A (slightly over-engineered) set of functions to control the Photon's built-in RGB LED.

rgb-control.ino

// Ernest Gu, 2016-06-17

void setup()
{
    Particle.function("setRGB", str_setRGB);
    Particle.function("setRGBcolor", str_setRGBcolor);
    Particle.function("setRGBmag", str_setRGBmag);
    
    // We need to take control of the RGB LED; there is no good way to share control with the
    // default firmware. (This might mean a substantial amount of reimplementation of functionality.)
    RGB.control(true);
}

void loop() { }

// setRGB() takes a command composed of a hex (RGB) value and a brightness (0-255), separated by ' '.
// TODO: If the input is ill-formed, the function returns -1 and no changes are made.
// Example input: "#BADA55 128", "#FFF 64"
int str_setRGB(String command) {
    
    String str_RGB = command.substring(0, command.indexOf(' '));
    String str_mag = command.substring(command.indexOf(' ')).trim();
    
    int err_color = str_setRGBcolor(str_RGB);
    if (err_color == -1) { return -1; }
    
    int err_mag = str_setRGBmag(str_mag);
    if (err_mag == -1) { return -1; }
    
    return 0;
}

int str_setRGBcolor(String command) {
    
    // Remove the optional '#' that indicates an HTML color hex code.
    if (command.startsWith("#")) {
        command = command.substring(1);
    }
    
    // Although HTML only allows #xxx and #xxxxxx, we accept any integer multiple of 3.
    if (command.length() % 3 != 0) { return -1; }
    int multiple = command.length() / 3;
    String str_RGB[3] = {"", "", ""};   // R, G, B
    
    // Split the string into its R, G, and B components.
    for (int i=0; i<3; ++i) {
        str_RGB[i] = command.substring(i*multiple, (i+1)*multiple);
    }
    
    // Convert each color's string into an int.
    int RGB[3] = {0, 0, 0};
    for (int i=0; i<0; ++i) {
        for (int digit=multiple-1, placeVal=1; digit>=0; --digit, placeVal *= 16) {
            
            // basic base conversion (hex -> dec)
            RGB[i] += placeVal * hexToInt(str_RGB[i].charAt(digit));
            
        }
        
        // scale the number into the range [0, 255]
        if (multiple > 2) {
            RGB[i] *= 2;
            RGB[i] /= multiple;
        }
    }
    
    return setRGBcolor(RGB[0], RGB[1], RGB[2]);
}

int str_setRGBmag(String command) {
    return setRGBmag(command.toInt());
}

int setRGBcolor(int R, int G, int B) {
    RGB.color(R, G, B);
    return 0;
}

int setRGBmag(int mag) {
    RGB.brightness(mag);
    return 0;
}

// Outputs 0 if invalid; changing to -1 might necessitate error handling. (TODO?)
int hexToInt(char ch) {
    if (ch >= '0' && ch <= '9') {
        return ch - '0';
    }
    if (ch >= 'a' && ch <= 'f') {
        return 10 + ch - 'a';
    }
    if (ch >= 'A' && ch <= 'F') {
        return 10 + ch - 'A';
    }
    
    return 0;
}