I think that's how it used to be initially, and it was possible to friendly fire kill people. For example, say you were on red, shooting at a blue, then somebody killed you, and you switched to blue, but your red paint (still in the air) hit a blue person. It would display that you killed a teammate, since you're now blue. I think that was a bug back in the day, and I fixed it by switching all the paint when a player changed teams, but that introduced this bug where you could switch teams and kill what used to be your teammates. I think the fix-all solution will just be to remove all paint from a player when he switches teams.