encodeRGB.js

function encodeRGB(r,g,b) {
    return encode_triplet(0,r,g)+encode_triplet(b,255,255);
}

function encode_triplet(e1, e2, e3) {
    var keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    enc1 = e1 >> 2;
    enc2 = ((e1 & 3) << 4) | (e2 >> 4);
    enc3 = ((e2 & 15) << 2) | (e3 >> 6);
    enc4 = e3 & 63;
    return keyStr.charAt(enc1) + keyStr.charAt(enc2) + keyStr.charAt(enc3) + keyStr.charAt(enc4);
}

gifrgb.less

.gifrgb(@r,@g,@b) {
    @k: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
    @g1: "url(data:image/gif;base64,R0lGODlhAQABAPAAA";
    @g2: "///yH5BAAAAAAALAAAAAABAAEAAAICRAEAOw==)";
    @b64img: ~`function(r,g,b){var k=@{k};return @{g1}+k.charAt(((0&3)<<4)|(r>>4))+k.charAt(((r&15)<<2)|(g>>6))+k.charAt(g&63)+k.charAt(b>>2)+k.charAt(((b&3)<<4)|(255>>4))+@{g2}}(@{r},@{g},@{b})`;
    background-image: @b64img;
}

div.progressitem {
  .gifrgb(255,255,0);
}