Suits are usually white, at least real ones are. I've seen silver ones in films and tv shows tho! The real ones are made from kevlar which is a bullet proof material. Probably white to deflect heat from direct sunlight? If there are silver ones in real use, perhaps the same reason.
As mentioned to reflect heat from the sun - there is no atmosphere in space so it's more severe. The metallic might also protect against radiation/particle rays?