Hello again!
We have discussed this idea and unfortunately we are going to have to decline this. We really did love the idea of rainbow wand effects but we just aren’t able to implement something like this. The only particle effect as far as we know that can be randomized to actually produce a rainbow effect are redstone particles. Unfortunately, redstone particles tend to exist after being used for a long period of time which can ultimately cause performance issues. Another issue with redstone is that they aren’t able to move up or down and just stay in the same spot; this would overall look very weird when comparing it to the other wand effects.
We really appreciate your suggestion, but because of these reasons we will be declining this idea. Thank you so much for bringing this suggestion to us and we hope to hear more from you in the future! Hope you have a fantastic day!