Under normal circumstances, the light shining on an apple is white light, such as daylight — light consisting of all the visible colors of the spectrum. Some of the colors shining on the apple are absorbed, and some are reflected. For an object that is not emitting light, but reflecting it, the balance between what is absorbed and what is reflected determines the color we see.
In the case of a red apple, the apple skin is absorbing the blue and green, and reflecting red. Similarly if a lemon appears yellow in daylight, it's because it is reflecting yellow (which is a combination of red and green light) and absorbing blue.