pixel-editor/js/_createColorPalette.js

function createColorPalette(selectedPalette, fillBackground) {
    //remove current palette
    colors = document.getElementsByClassName('color-button');
    while (colors.length > 0) {
        colors[0].parentElement.remove();
    }

    var lightestColor = '#000000';
    var darkestColor = '#ffffff';

    for (var i = 0; i < selectedPalette.length; i++) {
        var newColor = selectedPalette[i];
        var newColorElement = addColor(newColor);

        var newColorHex = hexToRgb(newColor);

        var lightestColorHex = hexToRgb(lightestColor);
        if (newColorHex.r + newColorHex.g + newColorHex.b > lightestColorHex.r + lightestColorHex.g + lightestColorHex.b)
            lightestColor = newColor;

        var darkestColorHex = hexToRgb(darkestColor);
        if (newColorHex.r + newColorHex.g + newColorHex.b < darkestColorHex.r + darkestColorHex.g + darkestColorHex.b) {

            //remove current color selection
            var selectedColor = document.querySelector('#colors-menu li.selected');
            if (selectedColor) selectedColor.classList.remove('selected');

            //set as current color
            newColorElement.classList.add('selected');

            darkestColor = newColor;
        }

    }

    //set as current color
    currentLayer.context.fillStyle = darkestColor;
}