If you have much time.. why dont you put more time in editing that tree first? Why dont you put time in studying how light works, search for general drawing tutorials etc?

Ok, now back on topic.
I think the rainbow can be improved a bit, but a boring rainbow will never get accepted, even if its the best rainbow there is! You will have to be more creative. The idea does not meet the quality standards.
If you really want to draw a rainbow, try making a scene with a rainbow in it! expand the idea, be creative!
But ok, if you want to improve this: remove the weird and random aa, add green and violet on the places they are needed, slow down the animation, remove the background, make it 8 colors max (rainbow = 7 colors + transparant background).

Now try that yourself. If you really have as much time as you say you have, you can experiment and try and try untill it looks great. Save the old versions, and edit them over and over. Compare, witch one looks better? Why? what looks worse? can I combine the good parts? etc.

i do suggest to quit this and focus on your tree for now. it's fine, but at the same time its nothing, just a cicle of colors and nothing else.

If you have time, why didnt you do what I said yet?