Detect similar colours from hex values

Question

Does anyone know of a way of taking two hex colour values and returning some kind of index to say how similar the colours are? e.g two shades of yellow might return a higher index i.e they are more similar than say a grey and yellow?

(Im using javascript but guess something like this would be a language independent formula/algorithm)

Answer 1

Here could be an algorithm to start with:

const yellow1 = "FFFF99";
const yellow2 = "FFFF00";
const blue = "0000FF";

function hexColorDelta(hex1, hex2) {
    // get red/green/blue int values of hex1
    var r1 = parseInt(hex1.substring(0, 2), 16);
    var g1 = parseInt(hex1.substring(2, 4), 16);
    var b1 = parseInt(hex1.substring(4, 6), 16);
    // get red/green/blue int values of hex2
    var r2 = parseInt(hex2.substring(0, 2), 16);
    var g2 = parseInt(hex2.substring(2, 4), 16);
    var b2 = parseInt(hex2.substring(4, 6), 16);
    // calculate differences between reds, greens and blues
    var r = 255 - Math.abs(r1 - r2);
    var g = 255 - Math.abs(g1 - g2);
    var b = 255 - Math.abs(b1 - b2);
    // limit differences between 0 and 1
    r /= 255;
    g /= 255;
    b /= 255;
    // 0 means opposite colors, 1 means same colors
    return (r + g + b) / 3;
}

console.log(hexColorDelta(yellow1, yellow2)); // 0.7999999999999999 
console.log(hexColorDelta(yellow1, blue)); // 0.19999999999999998