Actually it will have to wait, I'm still not exactly sure of what
you are after... as your snake grows the tail color  changes to yellow
as i would expect from the code...
---> did you want the tail yellow in the black and blue in the red
areas? if so, just set up a collision detection method  to determine
if the snake crosses into/out of the red boundaries and implement a
tail color change at the borders

Refinements,
keyDown() was deprecated to processKeyEvent()...so read up on the
class KeyEvent to keep your code  up to speed