it is not legal in my state to drive around with underglow lights on

First I've heard of that. There are some colors (blue or red) that would come under the "bad idea" category, but I'm not sure why they'd be illegal.

I had been fantasizing about a side-glow connected to turn signals, since on the Cruze, there's no way for someone who can't see the rear lights to see any turn signal indication. I'd also like a blinking underglow for the rear when backing up (think of a visual version of the backup beeper). I hadn't thought about any connection to the brake lights.

 

If your state laws are the same as what we have here in Saskatchewan, generally it's the coloring that makes the underglow lights illegal (some argue as well that it could be distracting for other drivers despite the color). The reasoning is:
Red - Could be mistaken for brake lights if viewed from the front.
White/Yellow - Could be mistaken for headlamps if viewed from the rear.
Blue/Green/Orange - Are used on emergency vehicles.

You could kind of get away if you had purple (black light) or pink as they didn't fall under the categories listed above. If you did get pulled over with these 2 colors you likely wouldn't have got a ticket, but if the officer told you to turn them off you wouldn't argue with them about it either.

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