The easy way would be to place a box behind your character, scale it so it covers everything and give it the correct color. You might have to prevent it casting a shadow. This is also the best way to display any picture in the background (make sure to size the box properly with the same proportions as the picture). The reason for not using a square is that it may display artifacts when rendering.

Don't forget to also color the ground or use the same trick there. Some special studio background props are available (free) that provide a gradual change from floor to background, these can be useful to prevent the harsh separation that may occur.

These tricks give more control over the rendering process.