Turning on Polypaint
On your right menu, scroll down till you see "polypaint". Turn on colorize.
Once that is on. Turn on your RGB and turn off Zadd or Zsub
Also add the color menu to your side bar for convenience
You are now ready to use paint on your model. Size,brushes and opacity are the same tools you would use for sculpting. ( In Alphas, draw size..etc)
But lets say you want to add paint on real skin texture.
Heres how:
Make sure your polypaint is turned on. Go into your "textures" menu and import a picture in.
Put the image into the scene on top of you model. Z to turn on the options and turn on the "paint" button
Turn on Spotlight projection.
and you are ready to paint on the skin. Paint like how you would normally but instead of just a solid color, Zbrush will project the image on to your model.
you can also press fillObject and it will fill the whole model with that image.
As you can see, the image is projected on to the model where i painted it in. Here's another example with an eyeball.
In conclusion, polypaint and spotlight projection is a great way to pay 2D textures to your models. It make make your model look more realistic and saves time. You can also export your polypaint to be used as a texture with your UV map.