I really like red as a bright contrasting color to put comments or circles on busy slides. Until a meeting with people in the Finance Department of one of my big corporate clients. "Can you please take the red off, in our (financial) language red equals bad news".
Three things to avoid in slide design:
  1. Bright red highlights in fonts, especially when talking about numbers
  2. Arrows pointing down (if you want to visualize something positive), why not redesign the slide and have them point up?
  3. Lines, sequences, or page elements that force the eye to go from top left to bottom right. The milestone graphic in this earlier post is a good example.

As always, these are not rules to be set in stone, it is just another piece of slide design context that you should be aware of.