Sort of relevant. Does anyone remember the hand colourising kits from years ago They were sold specifically for photos. My Mum had one, a box with several bottles of colours, very liquid, just like water, and some brushes. She spent several happy hours painting old photos with a very fine brush. I remember the flesh colour was quite good, made for a particular job. The others were generic blues, reds etc just to get as close as required. I don't think the colours were intended to be mixed, they were used straight from the bottle, perhaps overpainted to change shades.
Andrew