The base coat is that layer of paint which lies above the ground coat and the initial layers of modeling and under the final top coats.
For traditional painting the base coat should be a transluscent layer of colour broadly indicating the hues of each area of the painting in which it is to be used. It is transluscent to allow the underpainting to continue to show through where we have indicated our areas of shadow and light but opaque enough to begin to cover and hide the workings of our initial drawing.
The purpose of this layer is threefold:
1: So we can begin to see the colours which we have chosen for our composition and how they are going to work together and it is at this stage that we are still able to change our choice without undue affect to the final painting.
The base layer follows the principles of fat over lean and therefore should be fatter than the modeling layers underneath but leaner than those still to come. This is also why it should have transparency as in general a transparent layer is leaner and will dry faster than an opaque layer. Though this should be observed with a note of caution because if the transparent layer has been applied with a great deal of oil then it will be fatter.
So for the base layer do not use oil to thin your paint.
It is not always critical at this stage to be specific with our choice of colour but rather to use that which approximates what the final average hue might be when we take into consideration its light and shadow values.
2: However we can also use this layer to modify our final layers if we contiue to use transparency, particularly with reference to portrait and nude figure painting, vegetation, metals and drapery. The base layer can give extra depth and life to a painting by the use of these transparent layers. For instance we may use a highly saturated hue for our base layer and then begin to cover it with a layer of colour that is less saturated or vice versa to completely modify the final observed colour. As an example try applying a thin layer of Burnt Sienna and let it dry. Over this apply a similarly thin layer of Phalo Green. What you will get is an Olive green that has more depth to it than if you were to just mix an opaque olive hue on the palette.
3: As previously noted the base layer helps to hide our initial drawing and cover any errors we have made in that drawing or anything that we now wish to change or ignore.
Tip: Use a dry stiff old brush to scrumble the base coat with and save your expensive new brushes for the final layers.