This is a really fascinating subject that I was just trying to grasp while writing a trip report a few nights ago. I made a rather important personal discovery on aya relating to the "mind's eye", and it's role as a muscle that can be trained with definite results, for example being able to concretely imagine things into your surroundings and coming up with more out-of-the-box "objects" that aren't really based on anything you've previously observed, all of this even while sober. I say personal discovery because I have no idea if it will work for other people since I've never been another person. But maybe it will, anyway the theory itself is a bit offtopic so I'll go into it in more detail in the trip report.
But about coming up with new concepts or colors that aren't based on sensory input, it is very possible but it just seems to be hard most of the time because of evolutionary reasons relating to the survival of the host body, in other words, surviving and reproducing is alot easier when you're not constantly living in a magic world full of novelty, or "tripping". It currently seems to be more or less like a deactivated ability in us that psychedelics temporarily can activate. In everyday reality we're basically just reinforcing our known neural pathways, but as some aya studies have shown (
linky), it's easier to create completely new neural connections on aya, which is why for example conquering life-long phobias is easier on aya too. It's like deprogramming your old mental structures that have taken a good hold in your mind. It seems very hard most of the time as we're so routinely numbed by the same old neural patterns, but it is in my opinion very trainable and you can reach a degree of control over it while sober. The first important step is recognizing it (the process of new connections, and also the minds eye itself) in your mindscape, and psychedelics help with this initial step alot if you set your intent on it.
For example, try thinking about a completely new color like you mentioned. First you should remind yourself what colors are, they are just "markers" for your brain to distinguish different wavelengths from each other, so they only exist in your head. So why wouldn't it be possible to come up with new "markers"? Well it is, you just can't convey them to other people as there will be no agreement as to what natural phenomena should invoke the feeling of this new color. But the new color will exist in your head as a free concept. It obviously won't be as stable as all the known colors you've been mentally training with all your life, but it's still there, like a newborn baby color, waiting to be trained, and associated to specific phenomena if you so desire. It might be a bit tricky to come up with a new color at first , but think of it as extrapolating a curve, you have all these known values of known colors that make a curve, then go a bit left or right and try to extrapolate into the unknown. Recognize what it is that the old colors are "doing" and then make the same "doing"-action happen in a previously undefined area outside of the known curve. And this particular curve has nothing to do with the wavelengths themselves, it's more like a neural pattern curve, or "mental action" -curve.
When you get it right, you'll realize that there are in fact seemingly an infinite amount of new colors or "markers" that you can conjure up, and it's not only limited to colors or the visual sense. Now, if that's not exciting then I dunno what is.. So practice, practice, practice. Then some more practice.