Below's an online example which changes a pair of rectangles' color once
when they interpolate themselves; and get their default color back once they separate from each other.
There are no additional triggers (color changes) when they're together!

http://studio.processingtogether.com/sp/pad/export/ro.9owuz0wKx1ph7/latest

Here's another example:
http://forum.processing.org/topic/why-is-cursor-blinking-changes-from-arrow-to-hand#25080000002303589

- Read https://forum.processing.org/topic/where-to-place-threads-and-other-forum-practices please.
- Programming questions are for asking questions about code you show. You don't show any code, you just post your assignment, so I moved this here.
- We are not against asking for an assignment, but we strongly encourage to at least try a bit and to post your attempt.
- Your subject is useless. Nearly every message in this forum (in the Questions sections at least) is to ask for help. Imagine if everybody posted the same subject! (Zoho dated your subject in the URL because you are not the first to use it...)
- And avoid all caps subjects, no need to yell.