The setting of Reef Color Challenge is underwater, with a stack of colorful blocks in the middle of the screen. Above the stack sits one block that you control, and the trick is that you can change its color.
Match the block you control with the block directly below it. When the colors agree, a row disappears and the blocks above it move down. Clear as many rows as you can for the highest score, and watch the clock while you do.
Quick reactions are important, but the best players also think ahead: look for columns that already have your color on top and keep the tall columns under control. The deeper you dive, the harder the challenge gets. Color matching in a more traditional shape: Match 3 Jewels.