Illegal in Illinois - Driving a vehicle with a smoked or tinted lens or cover is also not permitted, as stated in Chapter 12, Section 12-212.It's not illegal. It's not about color, but intensity. Can it be seen? I have cops behind me all the time. And it can work fine. I use the Alla tailight/brake/turn signal combo LED bulb and it's brighter than the OEM bulb not tinted.
And this is just taillights. Brake lights are even brighter!
Rarely enforced. Engineer at my place of work received a ticket in 2022 while driving his WRX STI. Anecdotally only person I know ticketed for modified tail lights.