trello.js

// ==UserScript==
// @name         Trello Card Colour
// @namespace    http://tampermonkey.net/
// @version      1.0
// @description  Colours Trello cards according to their labels
// @author       Luke Pothier
// @match        https://trello.com/b/*
// @grant        none
// ==/UserScript==

window.onload = function() {
    'use strict';

    setInterval(function() {
        let cards = document.querySelectorAll("[data-testid='trello-card']");

        for (let i = 0; i < cards.length; i++) {
            colourCard(cards[i]);
        }
    }, 500);
}

function colourCard(card) {
    let spans = card.getElementsByTagName('span');
    let divs = card.getElementsByTagName('div');

    if (spans && spans[0] && divs && divs[0]) {
        divs[0].style.backgroundColor = pSBC(0.3, window.getComputedStyle(spans[0], null).getPropertyValue('background-color'));
        divs[0].style.borderRadius = '8px';
    }

    for (let i = 0; i < spans.length; i++) {
        if (spans[i].dataset.testid === 'compact-card-label') {
            spans[i].parentElement.parentElement.style.display = 'none';
        }
    }
}

const pSBC = (p, c0, c1, l) => {
    let r, g, b, P, f, t, h, i = parseInt,
        m = Math.round,
        a = typeof(c1) == "string";
    if (typeof(p) != "number" || p < -1 || p > 1 || typeof(c0) != "string" || (c0[0] != 'r' && c0[0] != '#') || (c1 && !a)) return null;
    if (!this.pSBCr) this.pSBCr = (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 = i(g), x.b = i(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 = i(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 = m((d & 255) / 0.255) / 1000;
            else x.r = d >> 16, x.g = d >> 8 & 255, x.b = d & 255, x.a = -1
        }
        return x
    };
    h = c0.length > 9, h = a ? c1.length > 9 ? true : c1 == "c" ? !h : false : h, f = this.pSBCr(c0), P = p < 0, t = c1 && c1 != "c" ? this.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 = m(P * f.r + p * t.r), g = m(P * f.g + p * t.g), b = m(P * f.b + p * t.b);
    else r = m((P * f.r ** 2 + p * t.r ** 2) ** 0.5), g = m((P * f.g ** 2 + p * t.g ** 2) ** 0.5), b = m((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 ? "," + m(a * 1000) / 1000 : "") + ")";
    else return "#" + (4294967296 + r * 16777216 + g * 65536 + b * 256 + (f ? m(a * 255) : 0)).toString(16).slice(1, f ? undefined : -2)
}