Apples are such a joy to paint - all those complementary colours working together which only make each other more vibrant.
I always keep the basic colour wheel in mind when I paint. Sometimes to achieve contrast, you don't necessary need to adjust the tone - which I think can be more instinctive, but one can juxtapose the opposite colour to find exciting results.
The following colour wheel reminds us of the theory, placing the primaries; red, yellow and blue with their secondaries colours that are achieved when mixed together; orange, green and purple. The opposites are the complementary colours.
The reason why I find these complementaries so vital is that when they are placed together, they enhance their opposite like no other, for example red will make green seem even greener and will give a more balanced palette. Think of red berries on a sprig of holly, a beautiful crisp Autumn day with a cobalt blue sky and bright orange trees, or the yellow throat of a fabulous purple iris. These partnerships are wonderful. The Impressionists found that a little of the complimentary colour applied to the shade of an object will lift it too. A green bush, for example can be improved with a dark maroon worked into the shade. Monet said: “Colour owes its brightness to force of contrast rather than to its inherent qualities … primary colours look brightest when they are brought into contrast with their complementaries.” You can of course mix the complementaries on the palette to obtain lovely rich greys. If you think about it, you are really borrowing colours from the whole palette, just in varying ratios and subtleties.
Apples, a few details -