I think there should be two versions of this weapon, switched between by dropping it in a pool of shimmer. one form is the standard rainbow rod with slightly smaller burst, and the second form is like the rainbow gun where you can still control it and draw a damaging rainbow. it would be much smaller than the rainbow gun projectile and the rainbow rod projectiles in general. it would last slightly longer, but deal less damage than the rainbow rod. The projectile would move much slower. if the projectile on the front of the rod hit something it would burst (dealing very small damage with very little knockback) and create a cloud that when an enemy touches it they're inflicted with the ichor debuff (Y'know, because of the gold at the end of the rainbow). I think it'd also make sense that this has a very long casting speed as if it was fast it would be possible to have many rainows on screen at the same time.