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Looks like the images are desaturated quite a bit, and then painted over, probably setting the painted layer to multiply. So for the hair (in the first 3 examples anyways) you don't even have to worry about creating a finicky hair selection, just colour roughly in the boundary of the hair, then set layer blend mode to multiply or color etc.
Wow - looks great! Who doesn't love Audrey Hepburn. It must be a too grey / muddy underneath the yellow. Underneath the paint layer, and overtop of the image, try adding a curves or levels layer and adjusting so the the light area where the yellow is goes completely white, and make sure to leave your black black. You would need to use a layer mask so you don't wash out the skin. Then paint with pure rgb yellow. Hopefully that works.