I follow Michael Chesley Johnson's blog and saw this the other day. I thought he had some good ideas about mixing grays and wanted to share them. I hope you find this as interesting as I did.
You can check out his work at http://www.michaelchesleyjohnson.com
Greys are both easy and difficult to conjure up. Easy, because there's nothing like a dirty brush to work its black magic in creating rather ugly greys. Difficult, because a pretty grey takes a certain amount of apprenticeship in mixing color.
First, let's make sure we've got reasonably clean brushes. That will keep you from summoning grey without meaning to. Now, let's think about how greys are made.
They say you can make a grey by mixing a color with its complement. This is true, but it can be a very muddy grey. A prettier grey can be made by mixing a color with its near-complement instead. This is because the grey is closer in character to the color being greyed. Try it. Use a color wheel to help you identify the near-complement. If you want to grey down a green, don't use red - instead, use red-violet or red-orange.
Let's take this a step farther. Look at the color you want to grey and decide if it is a cool or warm version of its base color. To grey it, add the same temperature of its complement. If it's a cool red, use a cool green. If you use a warm green with a cool red, this will make mud. Using a cool with a cool will make a more beautiful grey. As an example, I paint a lot of fog, and many times I'll start off with a light pink - that's cadmium red light with lots of white, and very cool - and then scumble on a light cool green, such as viridian with lots of white. This combination gives me a mudless fog.
In the little 5x7 sketch above, I use this approach, but for a sunny scene. The scene had a lot of grey in it. I painted all the major shapes with the complement of the correct value and correct color temperature, and then overlaid them with the local color.
(First posted June 15, 2011)
--- Michael Chesley Johnson, AIS PSA MPAC PSNM www.MichaelChesleyJohnson.com
Paint on my friends,