Painting color can be done in one of two ways: direct and indirect. Indirect painting is the process of glazing colors on top of one another to achieve a slow build-up. Like traditional some oil painters, many digital painters prefer this method to working 'directly'. If you want to learn about going from a grayscale underpainting to a fully colored image, check out "Grayscale to Color" in the store.
"Realistic painting" often means "realistic surfaces". If you want to improve the level of realistic detail in your paintings, this premium video (available in the store) is for you. It deals with two primary methods for adding detail: by hand with custom brushes, and with photo overlays. These methods are described in a very versatile way, and will work for any sort of illustration or concept art.
In art school, drawing 1 teaches you the basics of form, light, and shadow. This premium series approaches this fundamental subject-matter with Photoshop as the medium. Honestly, this is the foundation of painting, and if I could only recommend a single series from the store this would be it.