A few tips/notes:
*Use saturated colors to offset the gray, if you use regular it’ll look washed out. Sometimes it will look washed out anyway, fix it later.
*My method is similar to glazing in oil painting in that I use lots of layers of transparent colors. (Although, I don’t usually fully render in black in white before applying color, but it’s perfectly doable [however it’s more effort to get it to look natural imo and I prefer to paint on top of what I’ve done afterwards anyway])
*I don’t normally change the opacity of my brush at all, but I was feeling lazy so I did all this with a mouse
*Real working time was ~45mins, I originally meant to only have the starting multiply layer but I kept going back and doing more oops