Re: midle value colour
For the actual painting ('overpainting'...if that's a word), you mix the colors to get the the final value that you've determined you want. So if you did your 3-tone underpainting well, it should give a good foundation for the basic value structure...at least the major masses of light, middle, and dark.
Painting over an underpainting works differently based on how transparent your colors are in the 'overpainting'; transparent colors often get darker as they are layered over other colors, whereas the more opaque colors/color mixes are less altered by the color underneath, and you can paint them in more directly.
IMO the most important thing is choosing the values you want in the first place. You can always make adjustments to change the value later on if it's not right (except in watercolors where it's difficult or impossible to later adjust a color lighter). Hope this helped a wee bit.
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