Nothing groundbreaking but-

A function to convert RGB values to Hex

function rgbToHex(r, g, b) {
    var rgb = b | (g << 8) | (r << 16);
    return "#" + (0x1000000 + rgb).toString(16).slice(1);
}
Code language: JavaScript (javascript)

And a function that does the opposite, Hex to RGB

function hexToRgb(hex) {
    var shorthandRegex = /^#?([a-f\d])([a-f\d])([a-f\d])$/i;
    hex = hex.replace(shorthandRegex, function (m, r, g, b) {
      return r + r + g + g + b + b;
    });
  
    var result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
    return result
      ? {
          r: parseInt(result[1], 16),
          g: parseInt(result[2], 16),
          b: parseInt(result[3], 16)
        }
      : null;
}
Code language: JavaScript (javascript)

Not sure with whom to credit these but I’ve found myself needing to look them up enough that they’re now here for safe keeping