Hi Simon - I just tested it using one red gel on the light and then another identical red gel on top. It does look more saturated, however a quick test in Lightroom putting the cursor over the same area on both images reveals that while the overall light is reduced, the colour mix is the same. As a further test, I then upped the light output by one stop, still using both gels and this 3rd shot, is almost identical to the first one in both colour and intensity. Sorry for testing your drugged up brain - I got this wrong, and I'm wondering now how I came to that conclusion. (I possibly used two gels of similar, but not identical colour once and the results looked more saturated. Not sure tbh, The results match Steven's diagram). Hope your recovery goes well.