A lot of Photoshop "paintings" start out as hand-drawn sketches which are then scanned in and painted over in digital layers. Some of the coloring jobs are "transparent" and look like watercolor. But many more are done in "opaque" layers which look more like oil or acrylic paint. With the magic of Photoshop you can use both transparent and opaque layers in the same painting. I suppose you can do this with conventional paint too but in my experience it gets messy and takes a long time to dry.
I started with a brown line drawing made at "Old House" vineyards last week.
I used just two layers, one with the plain drawing and one with the colors. The colors are from memory; I didn't use a photograph. I'm still debating about whether to do all my winery studies in conventional non-digital media and whether to include in the collection, pieces which were done from photographs rather than on site.
Original drawing is Pitt brown ink technical pen, about 6" x 4", July 20, 2013. Click on the pics for somewhat larger view.