I've had 20% Lumunar on my current car for the last 10 yrs. No loss of color either. I've rarely any issues around Chicago and no issues in CA. The only thing about 20% is that it can be hard to see outside when there are no street lights. If you are in a rural area, I'd suggest going lighter. I have great night sight, but usually end up putting down my front windows when I'm on side roads in the middle nowhere. Urban night there's no issue.
Right now I'm toying with the idea of 35% on my 335i, but I don't like that it does appear a bit lighter with a light interior. I'm not sure what effect the auto dimming side mirrors with have. 20% + auto dimming could be dark.