But if you take a piece of paper and punch a hole in it and isolate that section of the landscape, you'll find it isn't as dark green as you thought. It's lighter, and grayer. Painters must be mindful of a great deal more than color. They are attentive to shape and proportion, the edges of compositional elements, the shape of the space around and between objects. Again, it's a matter of seeing what's there, not what one thinks should be there.
People bring their emotional responses to drawing.
When drawing faces, for example, they tend to make eyes too big and foreheads too small, because eyes have more emotional importance. A painter has to see the difference. Then there's the matter of figure and ground. Look at a coffee mug. Most of us pay little attention to this second sort of space, but a painter must.