Wikipedia

I think their contention was that it was an established thing that this sort of lighting was bisexual.

According to BOWIE Creators, the concept of bisexual lighting was invented in 2014 by a Tumblr fan of Sherlock who believed that the lighting was being used to signal that Dr. Watson was bisexual and would eventually be in a romantic relationship with Sherlock Holmes.

Based on the history tab it seems more like an Urban Dictionary thing. The examples section has this:

Lara Thompson, a lecturer of film at Middlesex University, has argued that bisexual lighting is not well-known, stating: "I would have to see more examples before I see bisexual lighting as a wholly convincing phenomenon".

It seems to me like a form of symbolism that has happened a few times based on the wiki article rather than an established trope.

Well, technically it’s just a wave.