Alright, call me a philistine, but I’ve always had the notion that colors were mixed on a palette and then applied directly to the surface to be painted, in basically a “paint by numbers” concept (OK, I know that’s a gross oversimplification – I mean, I don’t think sfumato can be achieved by plopping colors down in discrete chunks). But then, I’ve heard several artists say, in describing their own works, that the final colors we see are the result of layering paint on top of each other.
Eh? How does this work? Wouldn’t the top layer just cover up all the layers underneath? If not, how does one know how the colors will interact? (Or, why not just mix it on the palette before committing it to canvas?) Or is that where the “art” comes from?
It depends on what you’re painting with.
Watercolors are transparent, what you see is the pigment on top of the white of the paper. Once it’s down, it’s there. Let’s say you mix a color for a shadow of an orange, and put it down. After its dry (or you can work wet-on-wet) you may go back in and lay another hit of this color (mixed cooled down) to darken just the area in the shadow where the orange is touching the table. It adds more detail and more solidly sets the orange on the table. If you don’t do this carefully though you end up with bands of color instead of a smooth gradient.
I haven’t worked as much in oils, but I do know they can be used in many different styles: Thiness/opacity, texture by laying down paint, Laying it down and taking it off again later. It allows for smoother transitions between light and dark. You can paint the iris of an eye in thinned out color, and then add the light highlight in the pupil and the dark edge of the iris later. Thinning it out keeps the ‘watery’ texture of the eye, the additional hits allow you to sharpen and add detail.
You can’t really just mix it all up in the pallete beforehand and put it down because then you’ll just end up with women with round splotchy cheeks instead of a healthy glow. There’s a lot more thought going into it than a lot of people realize.
Like miamouse already said, watercolors are all about the layers.
Acrylics are just about impossible to layer. They’re opaque, so one layer just completely obscures the one beneath it. With those, you have to pre-blend your paints, and be quicker than hell with your application. It’s never been one of my strong suits and I’m really impressed by people who can work in it well.
Oil paintings are probably what get the most layers. Generally, you mix up the colors you’re planning on using a large amount of, and squirt out some extra base colors on your pallette. But when it comes to putting all the colors together into a coherent image, you have to apply/blend/shade right on the canvas. You can use paint thinner, linseed oils, or other mediums to help thin, thicken, and “wet” your paints so they don’t dry before you’ve completed blending.
A good sized oil painting can take up to a month or so to dry, if that tells you anything!