You make a new layer on top of that.
Have you ever worked with oil paints before?
Well the process is similiar only on the computer.
Use pure color, don't have it darkened or anything. And color it in on that new layer. once you have you can do one of two things. Go into filters for that layer and find multiply (if you did it right it should look nice)
Or go into opacity and lower it down to about 75-85.
I recomend Multiply.