That's a matter of opinion. if you get a white case it will reflect light and help illuminate creating a veritable lighthouse out of your gaming tower. If you want a more understated glow, you would go with darker colored cases that don't match the color of the leds.
Additionally consider the color scheme of your setup. If you already have case fan leds, GPU lights, gaming keyboard/mouse maybe you want to match those colors. Maybe you want to show off a little national pride and go with your flag colors?
Really, it's your case, just make sure you keep illumination within comfortable boundaries and you should be fine if you don't pick colors you find ugly.