I mean, you take one look at Greek statues and Roman busts and you realize that people figured how to aim for realism, at least when it came to the human body and faces, over 2000 years ago.
Yet, unlike sculpture, paintings and drawings remained, uh, “immature” for centuries afterwards (to my limited knowledge, it was the Italian Renaissance that started making realistic paintings). Why?
Most people think they see color, light, and shadow well enough. But they don’t. They know what color a thing should be, or what they perceive the color to be, and so they can’t see the way that the color really is. I think that part of the genius of a painter like Lucien Freud was that he was showing you the colors are they really are (…kinda of…), rather than the way people think they are. Highlights on a face aren’t just going to be lighter; they’re going to have different hues, depending on your light source. Flattening colors out to black and white seems easier, until you realize that you can have two wildly different colors that have almost identical values, and so you have to introduce some unnatural contrast in order to make a distinction between objects. Hell, B&W in general requires increasing contrast and fucking around with your virtual white and black points, or else your drawing looks flat and lifeless.
Photography–particularly film photography, where you don’t have software interpreting the image–can be a useful tool in seeing this. Without any filters, you can examine detail areas of an image and see how reflected light, and how shadows, are changing the hue of what you’re seeing. Your brain automatically makes adjustments, unless you’re really looking. And training yourself to really see what’s actually there, versus what you expect, is a very challenging process.
Yes, that’s my point. By “most people can see color well enough” I mean most people aren’t color blind and can tell apart basic color differences. That’s all.