I'm considering making a rainbow sprinkle cake, but would have to make it a day (or two) in advance of the party.  Wondering if the color in the jimmies would run on the buttercream - or if I keep the cake in the refrig before delivery, would that help?  Would appreciate advice.

I don't think they would, especially if you keep it in the fridge. Maybe you could put some icing on a plate or something and put some on it to test it out before doing it on the cake.

No, they won't run. The purple sprinkle cake in my gallery was iced in buttercream then jimmies (not the small round non-pariel sprinkles) were added to the sides and top.

Very much appreciated!  These comments are very helpful!