Red, I correct him. Red, he will nod in agreement. After scribbling for a while, he will announce, "Green aachi" (Green finished).
Other random colours he will throw in are : eyyo, purple, back, pink, white.
I taught him to draw the outline of a palm placed on paper - he has slightly modified this activity: Place palm on paper and draw ON THE PALM.
Try that, it's good fun!