When I'm coloring a drawing in PS, if I set the opacity to 50%, color, then lift my brush, then nick the same area, it stacks the opacity for a more opaque results. I want it to all be the same unless I increase the opacity, and I see this in a lot of YouTube videos. Right now coloring is nigh on impossible because all the colors stack and I have a weirdly shaded photo just from coloring.
As @Scott mentions, I'm not sure you have any options in Photoshop for a brush not to 'stack'.
An alternative would be to use layer opacity instead. You set the layer to 50%, for example, and the brush to 100%. That way you can paint without any overlapping.
Masking out the different areas and colouring them on different layers might help with your problem. More layers -> more control. You can always drop opacity and/or delete overlapping colours that way.
I find that while drawing and painting, the more lazy you get and work on one layer the more problems you have later on =)