So what happens to the color black in sunlight? It can appear very bleached out, so our model wore white shorts to contrast with the black t-shirt, and sat in half sun/half shade. This pose allowed us to study the rule stating that "The lightest dark cannot be darker than the darkest light". In other words, the sunlit side of the black t-shirt (the lightest dark) cannot be painted to appear darker than the shadow side of the white shorts (which are the darkest light). It sort of sounds like a tongue twister the first time you hear it, but seeing, comparing, painting this subtle difference can make or break a painting.
"Establishing the two most extreme values as soon as possible helps me take note of all other values that will fall somewhere between them"...Kenn Backhaus