AnswerMy guess: so that they are maximally visible outside of the [shuttle, ship, whatever].
NASA gives two reasons.
1. White reflects the light off of the astronauts suits keeping them cooler.
2. White also stands out against the dark backdrop of space so that other astronauts can spot them easily while they are outside working on the spacecraft etc.
This does not belong to NASA
1. so preparing for the mcat, I ran into a question about white astronauts wearing white, it mentioned the fact that while white reflects radiation better it also radiates less of it. I am not quite sure how this is supposed to keep the astronaut warm, does it also reflect the heat from the body back toward the skin and absorb less, while at the same time radiating less of what had been absorbed. ??