Javascript Color Class

Color.js

var Color = function() {
    this.r = this.g = this.b = 0;
    this.h = this.s = this.l = 0;
    this.a = 1;
};
/** RGB */
Color.prototype.cssRGB = function() {
    return "rgb("+Math.round(255*this.r)+","+Math.round(255*this.g)+","+Math.round(255*this.b)+")";
};
Color.prototype.cssRGBA = function() {
    return "rgba("+Math.round(255*this.r)+","+Math.round(255*this.g)+","+Math.round(255*this.b)+","+Math.round(this.a)+")";
};
Color.prototype.red = function() { return this.r; };
Color.prototype.green = function() { return this.g; };
Color.prototype.blue = function() { return this.b; };
/** HSL */
Color.prototype.cssHSL = function() {
    return "hsl("+Math.round(360*this.h)+","+Math.round(100*this.s)+"%,"+Math.round(100*this.l)+"%)";
};
Color.prototype.cssHSLA = function() {
    return "hsla("+Math.round(360*this.h)+","+Math.round(100*this.s)+"%,"+Math.round(100*this.l)+"%,"+Math.round(this.a)+")";
};
Color.prototype.hue = function() { return this.h; };
Color.prototype.saturation = function() { return this.s; };
Color.prototype.lightness = function() { return this.l; };
/** HEX */
Color.prototype.cssHEX = function() {
    return "#" + 
        (255*this.r < 16 ? "0" : "") + Math.round(255*this.r).toString(16) +
        (255*this.g < 16 ? "0" : "") + Math.round(255*this.g).toString(16) + 
        (255*this.b < 16 ? "0" : "") + Math.round(255*this.b).toString(16);
}
/** Transparency */
Color.prototype.alpha = function() { return this.a; };
/** Modifiers */
Color.prototype.saturate = function(v) {
    if("string" == typeof v && v.indexOf("%") > -1 && (v = parseInt(v)) != 'NaN')
        this.s += v/100;
    else if("number" == typeof v) // range 255 
        this.s += v/255;
    else throw new Error("error: bad modifier format (percent or number)");
    if(this.s > 1) this.s = 1; else if(this.s < 0) this.s = 0;
    Color.Convertor.HSLToRGB.apply(this);
};
Color.prototype.desaturate = function(v) {
    this.saturate("-" + v);
};
Color.prototype.lighten = function(v) {
    if("string" == typeof v && v.indexOf("%") > -1 && (v = parseInt(v)) != 'NaN')
        this.l += v/100;
    else if("number" == typeof v) // range 255 
        this.l += v/255;
    else throw new Error("error: bad modifier format (percent or number)");
    if(this.l > 1) this.l = 1; else if(this.l < 0) this.l = 0;
    Color.Convertor.HSLToRGB.apply(this);
};
Color.prototype.darken = function(v) {
    this.ligthen("-" + v);
};
Color.prototype.fadein = function(v) {
    if("string" == typeof v && v.indexOf("%") > -1 && (v = parseInt(v)) != 'NaN')
        this.a += v/100;
    else if("number" == typeof v) // range 255 
        this.a += v/255;
    else throw new Error("error: bad modifier format (percent or number)");
    if(this.a > 1) this.a = 1; else if(this.a < 0) this.a = 0;
    Color.Convertor.HSLToRGB.apply(this);
};
Color.prototype.fadeout = function(v) {
    this.fadein("-" + v);
};
Color.prototype.spin = function(v) {
    if("string" == typeof v && v.indexOf("%") > -1 && (v = parseInt(v)) != 'NaN')
        this.h += v/100;
    else if("number" == typeof v) // range 360 
        this.h += v/360;
    else throw new Error("error: bad modifier format (percent or number)");
    if(this.h > 1) this.h = 1; else if(this.h < 0) this.h = 0;
    Color.Convertor.HSLToRGB.apply(this);
};
/** Debug */
Color.prototype.toString = function() {
    return "<span style=\"color: "+this.cssRGB()+"\">"+this.cssRGB()+"</span> / <span style=\"color: "+this.cssHSL()+"\">"+this.cssHSL()+"</span> / <span style=\"color: "+this.cssHEX()+"\">"+this.cssHEX()+"</span> / alpha: "+this.a+"";
};

Color.makeRGB = function() {
    var c = new Color(),
        sanitized;
    if(arguments.length < 3 || arguments.length > 4)
        throw new Error("error: 3 or 4 arguments");
    sanitized = Color.Sanitizer.RGB(arguments[0], arguments[1], arguments[2]);
    c.r = sanitized[0];
    c.g = sanitized[1];
    c.b = sanitized[2];
    if(arguments.length == 4) c.a = arguments[3];
    Color.Convertor.RGBToHSL.apply(c);
    return c;
};

Color.makeHSL = function() {
    var c = new Color(),
        sanitized;
    if(arguments.length < 3 || arguments.length > 4)
        throw new Error("error: 3 or 4 arguments");
    sanitized = Color.Sanitizer.HSL(arguments[0], arguments[1], arguments[2]);
    c.h = sanitized[0];
    c.s = sanitized[1];
    c.l = sanitized[2];
    if(arguments.length == 4) c.a = arguments[3];
    Color.Convertor.HSLToRGB.apply(c);
    return c;
};

Color.makeHEX = function(value) {
    var c = new Color(),
        sanitized;
    Color.Validator.checkHEX(value);
    if(value.length == 3) {
        sanitized = Color.Sanitizer.RGB(
            parseInt(value.substr(0, 1) + value.substr(0, 1), 16),
            parseInt(value.substr(1, 1) + value.substr(1, 1), 16),
            parseInt(value.substr(2, 1) + value.substr(2, 1), 16)
        );
    } else if(value.length == 6) {
        sanitized = Color.Sanitizer.RGB(
            parseInt(value.substr(0, 2), 16),
            parseInt(value.substr(2, 2), 16),
            parseInt(value.substr(4, 2), 16)
        );
    } else throw new Error("error: 3 or 6 arguments");
    c.r = sanitized[0];
    c.g = sanitized[1];
    c.b = sanitized[2];
    Color.Convertor.RGBToHSL.apply(c);
    return c;
};

Color.Sanitizer = {
    RGB: function() {
        var o = [];
        if(arguments.length == 0) return;
        for(var i = 0 ; i < arguments.length ; i++) {
            var c = arguments[i];
            if("string" == typeof c && c.indexOf("%") > -1) {
                if((c = parseInt(c)) == 'NaN')
                    throw new Error("Bad format");
                if(c < 0 || c > 100)
                    throw new Error("Bad format");
                o[i] = c/100;
            } else {
                if("string" == typeof c && (c = parseInt(c)) == 'NaN') throw new Error("Bad format");
                if(c < 0) throw new Error("Bad format");
                else if(c >= 0 && c < 1) o[i] = c;
                else if(c >= 1 && c < 256) o[i] = c/255;
                else throw new Error("Bad format");
            }
        }
        return o;
    },
    HSL: function() {
        if(arguments.length < 3 || arguments.length > 4) throw new Error("3 or 4 arguments required");
        var h = arguments[0],
            s = arguments[1],
            l = arguments[2];
        if("string" == typeof h && (h = parseFloat(h)) == 'NaN') throw new Error("Bad format for hue");
        if(h < 0 || h > 360) throw new Error("Hue out of range (0..360)");
        else if(((""+h).indexOf(".") > -1 && h > 1) || (""+h).indexOf(".") == -1) h /= 360;
        if("string" == typeof s && s.indexOf("%") > -1) {
            if((s = parseInt(s)) == 'NaN')
                throw new Error("Bad format for saturation");
            if(s < 0 || s > 100)
                throw new Error("Bad format for saturation");
            s /= 100;
        } else if(s < 0 || s > 1) throw new Error("Bad format for saturation");
        if("string" == typeof l && l.indexOf("%") > -1) {
            if((l = parseInt(l)) == 'NaN')
                throw new Error("Bad format for lightness");
            if(l < 0 || l > 100)
                throw new Error("Bad format for lightness");
            l /= 100;
        } else if(l < 0 || l > 1) throw new Error("Bad format for lightness");
        return [h, s, l];
    },
};

Color.Validator = {

    /**
     * Check a hexa color (without #)
     */
    checkHEX: function(value) {
        if(value.length != 6)
            throw new Error("Hexa color: bad length");
        value = value.toLowerCase();
        for(i in value) {
            var c = value.charCodeAt(i);
            if( !((c>=49 && c<=57) || (c>=97 && c<=102)) )
                throw new Error("Hexa color: out of range for " + value + " at position " + i);
        }
    },
};

Color.Convertor = {

    /**
     * Calculates HSL Color
     * RGB must be normalized
     * Must be executed in a Color object context
     * http://mjijackson.com/2008/02/rgb-to-hsl-and-rgb-to-hsv-color-model-conversion-algorithms-in-javascript 
     */
    RGBToHSL: function() {
        //     
        var r = this.r,
            g = this.g,
            b = this.b,
            max = Math.max(r, g, b), min = Math.min(r, g, b);        
        this.l = (max + min) / 2;    
        if(max == min){
            this.h = this.s = 0; // achromatic
        } else {
            var d = max - min;
            this.s = this.l > 0.5 ? d / (2 - max - min) : d / (max + min);
            switch(max){
                case r: this.h = (g - b) / d + (g < b ? 6 : 0); break;
                case g: this.h = (b - r) / d + 2; break;
                case b: this.h = (r - g) / d + 4; break;
            }
            this.h /= 6;
        }
    },

    /**
     * Calculates RGB color (nomalized)
     * HSL must be normalized
     * Must be executed in a Color object context
     * http://mjijackson.com/2008/02/rgb-to-hsl-and-rgb-to-hsv-color-model-conversion-algorithms-in-javascript
     */
    HSLToRGB: function() {
        var h = this.h,
            s = this.s,
            l = this.l,
            hue2rgb = function(p, q, t){
                if(t < 0) t += 1;
                if(t > 1) t -= 1;
                if(t < 1/6) return p + (q - p) * 6 * t;
                if(t < 1/2) return q;
                if(t < 2/3) return p + (q - p) * (2/3 - t) * 6;
                return p;
            };
        if(s == 0) {
            this.r = this.g = this.b = l; // achromatic
        } else {
            var q = l < 0.5 ? l * (1 + s) : l + s - l * s;
            var p = 2 * l - q;
            this.r = hue2rgb(p, q, h + 1/3);
            this.g = hue2rgb(p, q, h);
            this.b = hue2rgb(p, q, h - 1/3);
        }
    },
    
};

I added a test for some modifiers:
https://www.int2byte.de/public/plotboilerplate/tests/test_color.html?color=rgba(0,167,185,0.5)