color your flats like normal. i merge them all into one layer after i color. put it in a folder with your lineart and anything else (as you can see i put the gold hat trim in there as well)
clip a layer to your folder, and fill it with your selected color. i used this mid-tone indigo, but you could conceivably use any hue or shade depending on the mood and color scheme of the piece. i indicated the bean in red of the range of colors that i normally use.
set the layer to multiply and fringe. you can obviously go back and adjust the colors now. my opacity is usually set between 50-75%, depending on the effect i want.
begin erasing where you want your “highlights” to be. i usually use the default pen tool set to transparency (the checkerboard below the color swatch). i’ll sometimes blend out some parts of the shading (the bridge of the nose, etc). i’ll also erase the shading on the eyes but that’s obviously not necessary.
tada! we’re not done yet though. preserve the opacity of your shading layer.
plop on some colors, i used a pure aqua and a mid-tone blue, however like i said before you can conceivably use any colors depending on what effect you want.
blend those out.
make a new layer and clip it to your folder. set it to luminosity, and i’ll usually set the opacity between 5-50%.
plop on some glowy highlights. i used that pink shade and the default airbrush set to 20 density. erase the parts where it intersects (?) with another part. example below.
now i boost the colors. as you can see i added that rainbow smudge, the light blue, and light yellow, all set to overlay.