I tried these. Actually a cool alternative to what I had been doing.
The simple solution has always been turning up the CMY wheels on my desk in unison until the image turns darker. I use these in combination with ColFX 2 and 3 often as a way to "lighting optimize" dark projection content while busking.
The show I'm on now however, I had to dial down the content gamma for all the LED content but found I was screwed when I started taking IMAG to the LED. I actually tried a few of these tricks on my last leg and have been able to get some pretty cool colored vignettes of the IMAG over the show content.