It depends on what you mean by "color". If you say a color is a certain wavelength of light, then there are a lot of "colors" that no human being has ever seen. However, since we know about the electromagnetic spectrum, these frequencies of light are not unknown. We can "see" them with instruments, but those instruments just translate the unsee-able frequencies of light into normal frequencies that we can see.
If you restrict the definition of color to something that the human eye is capable of seeing, then we have to ask what you mean by "unknown." Estimates of how many colors the human eye can perceive range from 1 million up to 7 million. I'm sure that someone has seen all of them. Now, they don't all have names. But color names are not precise. "Blue" for instance, covers several thousand different shades. Each individual shade may have hundreds of slightly different variations. But someone looking at these shades would be pretty sure that it was all blue. So if by "unknown" you mean "unnamed" then there are colors that haven't been named yet. But, any given color is probably close enough to a named color as to make no difference.
So really, there are not any more new colors. Unless the human eye changes...
