Lets say there's a new cone that detects ultraviolet light, represented here with
U. Really, I think this would work with any frequency, I'm just picking ultraviolet b/c it's outside the range we can currently see, so its easier to understand it as a new colour. But in reality, it might have difficulty making it down to the retina, based on some things I was reading, so don't get too caught up on the U, it's a thought experiment.
Then we have these cones: R G B U
This gives us 8 new colours that we cannot currently see!
These are the combinations they could fire in, asterisk means that this colour is new, we've never seen it before:
R | G | B | U | Description