Some people have asked why I paint the background of my pet portraits red, then cover them up with a different color later. This is a great question!
A base layer is an initial layer of paint applied to a canvas, on top the the gesso, which serves as a base for subsequent layers of paint, this can also be called toning or underpainting. Some people use underpainting to define the color values for later in a painting, often using a monochromatic method to block out light and shadow. Due to the small size of my paintings, I just do a solid layer. The color of the base layer then can be optically mixed with the overpainting, without the danger of the colors physically blending and mixing.
I like to have a ground to work on that is not the stark white that the canvas starts as. White can be very distracting, a toned canvas allows you to see the colors you are using rather than white. Often the base layer will peek through thinner applications of the top colors, or in small spots between brushstrokes, which gives an effect that I enjoy.
I choose red for my pet portraits due to the fact that actually contrasts with most pets I paint, as most pets are not a bright red! The red can react with the top layers of more natural color tones that most pets are, creating little spots of interest and excitement. As a warm color the, cooler colors of most animals will appear to pop against the warm color.
However, even though I use red for pet portraits, it is really good to experiment with what base layer color works for you, different subjects do call for different base layer colors. There is no right or wrong color to use for your underpainting.