Pigments and paints and colors, oh my!
When it comes to color, more is not always better. More tubes of paint, more choices, more ways to go wrong. If you work with a limited palette and you get into trouble with color, then the number of ways to correct the painting is limited.
Claude Monet used a limited palette of only six to nine colors. And by 1886 had dropped black from his palette completely, preferring to create a chromatic black with alizarin crimson and viridian green. I'm a big proponent of mixing a black (chromatic black) rather than using a pre-mixed black from a tube. A chromatic black is richer and more complex than a pre-mixed black. The greys and neutrals that result from mixing a chromatic black with the other colors on your palette will also have greater depth and vibrancy.
Mixing a Chromatic Black
First, check to make sure that there is no black in any of the colors you are using. How to do that? Check the label for the pigment code. Most paint manufacturers list the pigment code on the label. If the paint you're using doesn't list the pigment code, then you are probably using a student grade paint without much pigment in it. Switch to professional grade paint, and you will get better results in your color mixing.
Here are a couple of my favorite recipes for mixing a chromatic black:
Basic Complementary Colors: Using your darkest green and your darkest red. Check the labels to make sure there isn't any black in the color. My preferred mixture is Golden Quinacridone Crimson and Sap Green.
Purple Black: This is one of my favorite mixtures for landscape painting. And as the Impressionists believed that there was no black in the shadows this color would have worked well on their palette. Mix your darkest blue and your darkest red. My favorite mixture is Golden Ultramarine Blue and Alizarin Crimson Hue. This mixture will visually appear black in mass tone and will look more purple in a glaze mixture.
Earth Color Black: Mix Golden Ultramarine Blue and Raw Umber. This mixture will be warmer than the purple-black mixture above.
Modern Primary Black: See the Youtube video below for this black mixture. One I use a lot.
Whenever you mix a chromatic black, it is helpful to make a few tint mixtures with Titanium White to get a feel for the tones of the mixture. It's also useful to mix a bit of the chromatic black into the other colors on your palette to get a broad range of shades of color that will harmonize in the painting.
No matter how you use color remember - less is more.
More color resources:
EFGS Computer Lab. An in-depth color compendium. Information on all aspects of color. You can really nerd out at this site.
The Color of Art Pigment Database. More nerdy color information.
My color mixing video on Youtube shows how to make black with three modern primaries.