Underpainting and Glazing
Underpainting is painting a "rough draft" painting and then going over it when it is dry with oil paints. Some artist will paint with just gray colors and dark colors for the underpainting to define the light areas and the dark areas better. They will try to "model" the forms and get a more 3-D look or realistic look. Other artist will use color for this instead of grays.
Many artist will use acrylic paints to paint the underpainting. The main reason is acrylic paint dries pretty quickly. Then when this underpainting is dry they will go over it with oil paints for the final painting.
You can build up your painting by painting several underpaintings before you apply the final coat of oil paint.
Another painting technique artists use is called "glazing". Glazing is painting transparent thin colours over a dried underpainting. The colors show through these transparent glazes and have a wonderful effect. Artists will build up layers of thin transparent paint. You have to wait for each layer of paint to dry before you can proceed to "glaze" with a new layer of thin transparent paint. Each "glaze" slightly modifies the colour of what has already been painted on the canvas. When you look at it the colours are automatically mixed optically giving a rich deep colour.
Glazing has the strongest impact on showing middle tones and dark colours. An example would be painting a water scene that has transparent glazes to show rocks or fish under the water. Another example would be painting water drops on flower petals or on a still life of fruit.
Oil paint comes in transparent, semi-transparent, and opaque. As you paint with oils you will learn which paint colours are transparent, semi-transparent, and opaque. All paint colours can be used as glazes, even opaque paints. Many artist will use the opaque paints as a glaze to paint mist or fog. You have to thin the paint right down so that it very smooth flowing.
As a beginner you should try to paint an underpainting and as you get to know the paints better you will learn which colours have the transparent qualities you can utilize in the glazing technique.
To learn more about how to determine which paints are transparent, semi-transparent, and opague go to FAQ and there is a opacity test you can do to figure it out. Some of the paint manufacturers will put this on the label as information.