You’ve got three basic colors : RED, BLUE and YELLOW.

You can mix two of them:
RED+BLUE=PURPLE
RED+YELLOW=ORANGE
BLUE+YELLOW=GREEN

Then you need to have the three basic colors on one single block in order to create a white block:
PURPLE + YELLOW = WHITE
RED + GREEN = WHITE
BLUE + ORANGE = WHITE

Once 3 white blocks are aligned, they disappear.