so first get some EHOLE, and then layer goo on top with different life and deco. If you put a square of 
GOO that is decoed red and has a life of 0 (never decays) and then put a square of GOO with a life of 300 (decays in about 5 seconds, depends on FPS), and is decoed yellow, then you will get an animation with a yellow square which turns red after 5 seconds.

If you don't put EHOLE, any long GOO animation will just turn into BHOL instead of making video.

There's a Lua script which does the thing for you.