Basically the game works like rainbow and gradients do. (really!)
When you select a "stage", it gives you a preview of the stage's tiles colors. (ex.: red, dark orange, light orange, yellow)
When you're on the playfield, notice the color of the cursor thingie. If it's red, you'll be able to shift the colors from yellow to red, and if it's yellow, you'll be to shift color from red to yellow. Depending of how many colors and which colors the stage offers, the cursor will randomly change between these colors.
Earlier stages are easy (4 colors), but later on you'll have a full rainbow of colors where selecting the wrong color shifting will result in locking the current tile.
Special 'X' tiles will also clear tiles in diagonals, and special 'O' tiles will clear all the tiles of the same color on the playfield.
This bg title offers lots and lots to unlock (Clear mode, playfield templates...), and is easily one of the best from the two series IMHO.
4cr says 'Hi'.