I'm trying to reproduce the old oil paint technique called grisaille. I followed along a tutorial meant for oil paints, but using photoshop instead. The technique uses seven layers of paint. For each layer, I made a layer in photoshop and used a few brushes with pressure sensitive opacity and flow. Each semi-transparent layer is affected by the colors used on the layers underneath, creating an interesting blending effect.
Here's the oil painting tutorial: [link]