I saw this done in a Youtube video.
1. Throw the barrel near the Necky when you're under the 3 bananas to the right of the Expresso token and keep chasing it.
2. When you pass Rambi, stop when you are overlapping him and jump on him.
3. The Gnawty that gets killed by the barrel should be golden.
The bananas will now change to cool colors and a bunch of other color glitches will happen. EDIT: I got lucky and did it right on the first try, not knowing how hard it was to do, so I edited this post to contain the save state, 1 picture, and a couple of ZMVs.