Thus, every color, shadow, and highlight had to be put there by hand . With experience you'll be able to confidently judge. Or confidently fake it, which is what most art-types do anyway. Each work of art has elements of unity and elements of variety. Some ways of creating unity might be to make everything in a painting a similar color, or to have a series of repeating shapes, or a consistent texture made with brush strokes.
Variety balances out unity and keeps things interesting.
At times, variety is used to cause the eye to pay particular attention to that object. Ask yourself why the artist would bother trying to single out your attention to that object.