OK, just so you know, I spell it "color." I'm American, that's how we spell it here. If you spell it "colour," please don't disregard this, the science and math is still correct. It's not that "I'm to stupid to spell 'colour' right," it's just that I'm from and live in a region that spells it "color."
A couple years ago there was a meme going around about "try to imagine a new color if you can do anything" or something ridiculous like that. Now this meme makes me angry. Why does it make me angry, because it's scientifically and mathematically impossible. So what makes it impossible? It's not like the human mind can't comprehend it or something like that, it's because every possible color already exists.
OK, lets look at the science behind color. Color is the result of different frequencies in the light wave. High frequencies light waves make your violet and blue colors and low frequencies make your red and orange colors. Every color has a specific frequency, and every frequency has a specific color. Of the electromagnetic spectrum, visible light is a relatively small section, but that's relative. The difference between any 2 colors is infinitely large. Which brings us to the math.
Now tell me, how many numbers are between "0" and "1"? The answer, infinitely many. There's 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9. Then how many numbers are between 0.1 and 0.2? Again, infinitely many. 0.11, 0.12, 0.13... and so on and so forth. Each frequency has a number value assigned to it. And just like with the numbers, there are an infinite number of frequencies between any 2 frequency value.
So what does this have to do with anything? Well you have an infinite number of numbers between any 2 numbers. Like numbers, there are an infinite number of frequencies between any 2 frequencies. Since frequencies are assigned to colors, that means that there's an infinite number of colors.
"So there's an infinite number of colors, why can't I think of a new color then?" Well that's a funny thing about infinity. You see, you can't "add to or subtract from" infinity. There are some mathematicians that try to figure out what happens when you add infinity to infinity, or subtract infinity from infinity, but they all agree that if you add or subtract 1 from infinity it will not have any affect on infinity. It will have an infinitesimal effect, but infinitesimal is just a fancy math word "zero with out actually being nothing." In layman's terms, absolutely no effect.
Another interesting point is that the visible light spectrum does have ends. There is an infinitesimally small difference between "invisible" infrared and "visible" red and another infinitesimally small difference between the "invisible" ultraviolet and the "visible" violet. With this in mind, there is in fact a very specific range for which visible light exists. Any frequency above or below this will be invisible to the human eye and will not be seen by a human. It's not like you could just get a new frequency because that range of frequencies are the only frequencies humans can see. And you can't "discover an unknown" frequency because it's already existed since the beginning of time.
So here's the thing, you will never imagine a new color. Computers will never imagine a new color. You would need to be able to see electromagnetic waves that have a lower or higher frequency that humans can see. But unless that happens, you're stuck with the visible light spectrum.
However, if you are dead set on "imagining a new color," you can always make a new name for a previously unnamed color. Talk to an art person, and they will name off colors you've never heard of but have seen. Remember what I said about "you can't add 1 to infinity and have any effect," well you can also not subtract 1 from infinity and have any effect. You can always find a previously unnamed frequency, since there's an infinite amount of frequencies but only a finite number of names. Well, have fun categorizing your newly named colors.