Tiniting tails is fine. Some cars out there have much deeper blood red tails compared to the bright red e9x. The key is to not go overboard like you said.
The best thing IMO would be a lamin-x film in a very light shade. The spray stuff can be tricky and conditions have to be perfect. I did my sidemarkers with Niteshades and I had to use a lot of coats to make it uniform, and it still has a rough blotchy looking texture.
Try the film first.
M3 Edition Grilles|Painted Reflectors|Matte Black Vinyl Window Trim|CF Performance Spoiler|35% Tint|19" Satin Black Avant Garde M359|Lux H8 V3
OEM Aero Lip|M3 Side Skirts|LCI Tails|M-tech Rear|Duke Dynamics CF Performance|Cyba 4.0 Black Chrome