SC2Mapster

This is pretty simple, and shouldn't take you long at all.

Okay, to start off, create the behaviors "Red" "Orange" "Yellow" "Green" "Blue" and "Purple." Set the duration of the behaviors to whatever you like. The shorter the duration, the faster the unit will go through the colors. Now, create apply behavior effects named "Apply Red" "Apply Blue" and so on and so forth, respectively.

Now set the "Effect - Final" of each behavior to the "Apply Color" of the next color in the rainbow. If you do not know what the order is, check the first sentence. (derp.) Next, apply one (just one!) of the behaviors to the unit you want rainbowing.

Now, for the actor events parts. Go to the actor of your rainbowing unit. Now double click on "Events - Terms." Make 6 new events in the new window that pops up. (Make new events by right-clicking the part of the window with alot of text.) Set the msg type of the event to "behavior" then source name to "Red" "Orange" or whatnot. Now, set the sub name to "start." The action part of the event should have the msg type be "Set Tint Color Blend." Then, set the first folor to the name of the behavior you set as the event, and the second color as the next color in the rainbow. Set the "blend in duration' to however long your behaviors are. Do this for the other events.

You should be done! See if it works :D

This actually contains exactly the information I was looking for a minute ago in the search function. Thanks :)