Creating a new color is impossible. Our eyes can only percieve such a set color range, only a few million colors. We can't create colors in our dream world and institute them into reality. You might be able to say, when I enter the next room I'll see a new color but when you wake up it'll probably be a color you've seen before. Or when you try to remember what it looked like, come up with nothing. Then there's the all important question: How would you describe it? I think you can equate this to the asking DCs to tell you the funniest joke ever. You'll hear it and think it's hilarious but when you wake up it's just nonsense. However, yes you can persuade yourself while dreaming that you saw a new color but in reality you didn't.