Ill just move an answer i wrote to here,
Q: How do you paint so dark? I always have to press really hard to have a dark color
A:
you can do it a few ways, you can just adjust your opacity and flow at the top toolbar while you have your brush selected, if you have black selected and you're having trouble it's most likely because one of these percentages is low, I usually work with opacity pen pressure enabled, opacity 100% and flow 40-100% depending on what I'm doing.
If you have PS CS3 or up I think, in your brush window (Top toolbar:Window > Brush) in the left menu, click Transfer, under opacity jitter if you have pen pressure enabled there should be a bar labeled 'Minimum:" you can slide that to adjust how much opacity the brush will be capable of painting, same applies for flow
if you have both, opacity and flow pressure enabled, disable one of them and it will be easier
If you still can't get it to work leme know