Black and silver (to a lesser extent) act to dim the aura of the glow. While the colour of the arm does not change, the distance of the coloured aura from the arm is decreased.
You can dye them with black but resulting color is just a washed out look of undyed. If you mix black with some other color you get darker, less-glowing version of gloves.
But really, you have chaos gloves and you can't afford a black dye to test it?
