Disclaimer: I have no experience with this drug class, and my medical knowledge is that of a novice.

“Don’t expose it to light” typically means “don’t leave it in the sun too long.”

These warnings usually mean that the chemical will denature / break down when exposed to light or heat which leads to a decrease in efficacy. Most chemicals will only do this in response to IR or UV light. If the reaction is from IR, then the primary worry is keeping it cool enough. By the nature of indoor lights, your bathroom light is almost certainly not emitting enough UV to cause a reaction.

If the drug is colored (I don’t believe this to be the case here), then visible light could cause a reaction, though I would still find it unlikely that a drug would lose sufficient efficacy due to a bathroom light.

I wouldn’t worry unless it got too warm for too long.

I can’t say anything about the label turning red without a bit more info about the container itself (is it just a vial?), but it sounds like it might be an indicator that you took your last dose.

As always, contact your doctor or provider if you have concerns.