Reposted from mjijackson.com -- this is so cool that we should make sure there are more copies of this on the net, just in case. =D

Here is a set of additive color model conversion algorithms that I found published on Wikipedia and have implemented in JavaScript. It was surprisingly difficult to find these actually implemented anywhere in compact, efficient, and bug-free code, so I wrote my own. These should be easily portable to other programming languages if desired.

/** * Converts an RGB color value to HSL. Conversion formula * adapted from http://en.wikipedia.org/wiki/HSL_color_space. * Assumes r, g, and b are contained in the set [0, 255] and * returns h, s, and l in the set [0, 1]. * * @param Number r The red color value * @param Number g The green color value * @param Number b The blue color value * @return Array The HSL representation */ function rgbToHsl(r, g, b){ r /= 255, g /= 255, b /= 255; var max = Math.max(r, g, b), min = Math.min(r, g, b); var h, s, l = (max + min) / 2; if(max == min){ h = s = 0; // achromatic }else{ var d = max - min; s = l > 0.5 ? d / (2 - max - min) : d / (max + min); switch(max){ case r: h = (g - b) / d + (g < b ? 6 : 0); break; case g: h = (b - r) / d + 2; break; case b: h = (r - g) / d + 4; break; } h /= 6; } return [h, s, l]; } /** * Converts an HSL color value to RGB. Conversion formula * adapted from http://en.wikipedia.org/wiki/HSL_color_space. * Assumes h, s, and l are contained in the set [0, 1] and * returns r, g, and b in the set [0, 255]. * * @param Number h The hue * @param Number s The saturation * @param Number l The lightness * @return Array The RGB representation */ function hslToRgb(h, s, l){ var r, g, b; if(s == 0){ r = g = b = l; // achromatic }else{ function hue2rgb(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; } var q = l < 0.5 ? l * (1 + s) : l + s - l * s; var p = 2 * l - q; r = hue2rgb(p, q, h + 1/3); g = hue2rgb(p, q, h); b = hue2rgb(p, q, h - 1/3); } return [r * 255, g * 255, b * 255]; } /** * Converts an RGB color value to HSV. Conversion formula * adapted from http://en.wikipedia.org/wiki/HSV_color_space. * Assumes r, g, and b are contained in the set [0, 255] and * returns h, s, and v in the set [0, 1]. * * @param Number r The red color value * @param Number g The green color value * @param Number b The blue color value * @return Array The HSV representation */ function rgbToHsv(r, g, b){ r = r/255, g = g/255, b = b/255; var max = Math.max(r, g, b), min = Math.min(r, g, b); var h, s, v = max; var d = max - min; s = max == 0 ? 0 : d / max; if(max == min){ h = 0; // achromatic }else{ switch(max){ case r: h = (g - b) / d + (g < b ? 6 : 0); break; case g: h = (b - r) / d + 2; break; case b: h = (r - g) / d + 4; break; } h /= 6; } return [h, s, v]; } /** * Converts an HSV color value to RGB. Conversion formula * adapted from http://en.wikipedia.org/wiki/HSV_color_space. * Assumes h, s, and v are contained in the set [0, 1] and * returns r, g, and b in the set [0, 255]. * * @param Number h The hue * @param Number s The saturation * @param Number v The value * @return Array The RGB representation */ function hsvToRgb(h, s, v){ var r, g, b; var i = Math.floor(h * 6); var f = h * 6 - i; var p = v * (1 - s); var q = v * (1 - f * s); var t = v * (1 - (1 - f) * s); switch(i % 6){ case 0: r = v, g = t, b = p; break; case 1: r = q, g = v, b = p; break; case 2: r = p, g = v, b = t; break; case 3: r = p, g = q, b = v; break; case 4: r = t, g = p, b = v; break; case 5: r = v, g = p, b = q; break; } return [r * 255, g * 255, b * 255]; }

via mjijackson.com