Javascript Random Dark Color

randomDarkColor.js

function randDarkColor() {
    var lum = -0.25;
    var hex = String('#' + Math.random().toString(16).slice(2, 8).toUpperCase()).replace(/[^0-9a-f]/gi, '');
    if (hex.length < 6) {
        hex = hex[0] + hex[0] + hex[1] + hex[1] + hex[2] + hex[2];
    }
    var rgb = "#",
        c, i;
    for (i = 0; i < 3; i++) {
        c = parseInt(hex.substr(i * 2, 2), 16);
        c = Math.round(Math.min(Math.max(0, c + (c * lum)), 255)).toString(16);
        rgb += ("00" + c).substr(c.length);
    }
    return rgb;
}

For a horizontal bar that will be number zero ["55", 0], how would I change the color function code structure so it shows no color in that one horizontal bar (while the rest show as normal with color)? I am using the light color bar code.
At present there is a tiny tiny slice of color on the far left when I use 0.
Here is a live example: https://codepen.io/crux1/pen/KKQZMzN