Using a basic palette of primary colours: Cadmium Red, French Ultramarine, Cadmium Yellow and Titanium White, paint a piece of fruit, an orange in this case.
Slice off some quarters of it to make the composition more interesting and shed some light on it. Use the complementary colour of the fruit to make the shadows on it (for help see the Colour Wheel in Techniques and Advice). Then choose a fun colour for the background.
For the shadows use the background colour but make it darker by mixing in some of its complementary colour (i.e if it is pink add green to it) but keep it expressive and loose by adding in some of the fruit colour which would reflect onto the surface. Don't worry about rough edged brushstrokes.
At the end add a few lines with the edge of your brush in stronger contrasting colours...green, blue or scarlet.