Source: https://github.com/PimpTrizkit/PJs/wiki/12.-Shade,-Blend-and-Convert-a-Web-Color-(pSBC.js)

lightenDarkenColor.js

/**
 * Source: https://github.com/PimpTrizkit/PJs/wiki/12.-Shade,-Blend-and-Convert-a-Web-Color-(pSBC.js)
 Usage:
 // Shade (Lighten or Darken)
pSBC ( 0.42, color1 ); // rgb(20,60,200) + [42% Lighter] => rgb(166,171,225)
pSBC ( -0.4, color5 ); // #F3A + [40% Darker] => #c62884
pSBC ( 0.42, color8 ); // rgba(200,60,20,0.98631) + [42% Lighter] => rgba(225,171,166,0.98631)
 */
static pSBC(p, c0, c1, l) {
    let pSBCr = function (d) {
        let n = d.length,
            x = {};
        if (n > 9) {
            [r, g, b, a] = d = d.split(","), n = d.length;
            if (n < 3 || n > 4) return null;
            x.r = i(r[3] == "a" ? r.slice(5) : r.slice(4)), x.g = parseInt(g), x.b = parseInt(b), x.a = a ? parseFloat(a) : -1
        } else {
            if (n == 8 || n == 6 || n < 4) return null;
            if (n < 6) d = "#" + d[1] + d[1] + d[2] + d[2] + d[3] + d[3] + (n > 4 ? d[4] + d[4] : "");
            d = parseInt(d.slice(1), 16);
            if (n == 9 || n == 5) x.r = d >> 24 & 255, x.g = d >> 16 & 255, x.b = d >> 8 & 255, x.a = Math.round((d & 255) / 0.255) / 1000;
            else x.r = d >> 16, x.g = d >> 8 & 255, x.b = d & 255, x.a = -1
        }
        return x
    };

    let r, g, b, P, f, t, h, a = typeof (c1) == "string";
    if (typeof (p) != "number" || p < -1 || p > 1 || typeof (c0) != "string" || (c0[0] != 'r' && c0[0] != '#') || (c1 && !a)) return null;

    h = c0.length > 9, h = a ? c1.length > 9 ? true : c1 == "c" ? !h : false : h, f = pSBCr(c0), P = p < 0, t = c1 && c1 != "c" ? pSBCr(c1) : P ? {
        r: 0,
        g: 0,
        b: 0,
        a: -1
    } : {
            r: 255,
            g: 255,
            b: 255,
            a: -1
        }, p = P ? p * -1 : p, P = 1 - p;
    if (!f || !t) return null;
    if (l) r = Math.round(P * f.r + p * t.r), g = Math.round(P * f.g + p * t.g), b = Math.round(P * f.b + p * t.b);
    else r = Math.round((P * f.r ** 2 + p * t.r ** 2) ** 0.5), g = Math.round((P * f.g ** 2 + p * t.g ** 2) ** 0.5), b = Math.round((P * f.b ** 2 + p * t.b ** 2) ** 0.5);
    a = f.a, t = t.a, f = a >= 0 || t >= 0, a = f ? a < 0 ? t : t < 0 ? a : a * P + t * p : 0;
    if (h) return "rgb" + (f ? "a(" : "(") + r + "," + g + "," + b + (f ? "," + Math.round(a * 1000) / 1000 : "") + ")";
    else return "#" + (4294967296 + r * 16777216 + g * 65536 + b * 256 + (f ? Math.round(a * 255) : 0)).toString(16).slice(1, f ? undefined : -2)
}