I've had the same problem ever since I updated my video drivers (I think a few months ago) to try and fix a problem with TF2 freezing/crashing on me. I tried removing and then reinstalling my video drivers but TF2 still kept crashing until I just completely removed my NVIDIA drivers. I think the Halloween spell problem started for me before I removed my NVIDIA drivers but I can't remember for certain. In any case, TF2 no longer crashes on me but I can't see Halloween paints shift colors or pulse. All of the other Halloween spells work properly for me, it's just the paint spells that don't work right. I can see the paint spells change colors one shade at a time every time I touch a resupply locker, switch my current weapon, or make an adjustment to an equipped weapon or cosmetic. I haven't heard anyone mention having the same problem as me so I figured I might as well just accept it for now until I can get around to trying to fix my video drivers again.