Hi, I am not sure if I understand your question correctly, but if you mean controlling lights which are off, or have 100% black color using LightMix, then it is not possible. The LightSelect elements are composed together using the ADD operation. If you have a completely black LightSelect element, adding it to others will result in no change whatsoever.
So the only solution to this is rendering with all lights ON (and preferably set to pure white color and similar intensities) and then turning them on, off, and changing their colors and values using LightMix.
TL;DR invisible lights do not have any color so you cannot change it.