In class you had some practice laying an even wash, while taking care to preserve a complex profile for the overall shape. To take that a step further, try the following exercise:
1) Make a pale wash over half or more of your paper, saving a large white shape within it.
2) When that wash is dry, make another of a different color, overlapping the first one. Within this second wash, save a shape that partly overlaps the reserved shape from the first wash (better read that part again). Let the paper dry.
3) Make a third layer now, with its own reserved shape.
The three washes will make new colors where they overlap, and they will show their simplest color where they appear on top of what was white paper. You could plan for the greatest number of colors, or the fewest, or simply follow your instincts. However you arrange your shapes, make your washes as smooth and even as you can, without correcting flaws.