I tinted only the backup lights, leaving anything red, red, but anything clear, black.
The OP's complaint seemed to be specifically about his backups, though, a pedestrian could not see them. I really haven't had that happen to me at all, the tint makes them look black when not lit but they still light up just as much as ever as far as I can tell.
If I had it to do over, though, I would still go with a film instead of a spray, seems like it would be much easier to remove and would guarantee a single level of tint, while it is easy to screw up with spray and get things uneven. Also, I suspect there's less work involved in trimming it to fit where you want (like with me, leaving all the red untinted) than there was in doing that same trim job with masking tape. I guess until I try it, though, I won't be really able to compare jobs.