just change the color in stroke() ...

http://processing.org/reference/stroke_.html

... according to the current frame or time:

http://processing.org/reference/frameCount.html

http://processing.org/reference/millis_.html

you might want to look at modulo too. use it to wrap values around:

http://processing.org/reference/modulo.html

if you have any specific code you need help with, please post it.

best,
F