To test a suspicion of how setting a custom White Balance would handle a strongly colored scene, I used a red light in a totally dark room. Image 1 is the shot of a white object I took in Auto White Balance, which I used to set a custom WB. Image 2 was taken using the Custom WB.

Note that in the Custom shot, the Red level is actually lowered, theoretically to try and "match" the green and blue levels, but obviously there is so little green or blue that the shot just has a low level of red actually showing. In the Lightroom histogram, there is green showing at about 1 2/3 shot lower than red (which is about 1/2 stop above "medium") and blue shows up at about 1/2 stop below green.

The last two shots are copies of the first two processed in Lightroom with White Balance and Hue sliders moved all the way to "cool" (blue) and green and with the red saturation slider moved all the way down.