what color is the color of nothing?

Answer 1

Our senses receive messages which are "translated" in messages in our brain. Then, our logic is making it knowledge and keeps information for further use.
Physically, color is the logical understanding of of irradiation, reflectance, ect., of light from bodies, which is received from our eyes, ...
Thus, nothing does not have color, itself.
Though, our logic might "create" color from nothing, not that it exists, but because the brain in that instant, for several reasons, is doing it!
This could be a long discussion, but we need preparation, special sitting arrangements and good quality wine!

Answer 2

We perceive color based on the wavelenths that are reflected off something to our eye. Scientifically, we perceive black because that is the presence of all color, and conversely, we perceive white from the absence of all color. Therefore, the closest answer to your question would be white is the absence of all color... or ... nothing...

Answer 3

Strictly speaking, there is no "color of nothing". If only existing entities can be colored, but nothing refers to non-being--that which does not exist--therefore, non-being cannot be colored. Therefore there is no color of nothing.

If you mean symbolically, I would say black, since black is associated with darkness, which is a privation of light.