Uncle_Bens

function hexToTriple(hex) {
    hex = hex.slice(1);
    if (hex.length === 3) {
        return hex.split('').map(byte => parseInt(byte.repeat(2), 16));
    } else if (hex.length === 6) {
        return hex.match(/.{2}/g).map(byte => parseInt(byte, 16));
    } else {
        throw new Error(`invalid color ${hex}`);
    }
}

function tripleToHex(triple) {
    return '#' + triple.map(byte => byte.toString(16).padStart(2, '0')).join('');
}

function hexColorSubtract(hexAColor, hexBColor) {
    const [tripleA, tripleB] = [hexAColor, hexBColor].map(hexToTriple);
    const resultTriple = tripleA.map((byte, index) => {
        return Math.max(byte - tripleB[index], 0);
    });
    return tripleToHex(resultTriple);
}

// использование:

hexColorSubtract("#cef", "#110011"); // #bbeeee

function hexColorSubtract(ca, cb) {
    const a = parseInt(ca.substring(1), 16);
    const b = parseInt(cb.substring(1), 16);
    return '#' + ('000000' + Math.max(0, a-b).toString(16)).slice(-6).toUpperCase();
  }
  
  hexColorSubtract("#cef1ff", "#110011"); // #BDF1EE