I recently had a conversation with another artist about choosing colors.  My woodcuts tend to have fairly simple color schemes, (2-5 colors), so the color choice is important.  Perhaps surprisingly though, those decisions tend to happen fairly late in the process.  For much of the time that I am working on an image, I am treating it as a greyscale image, concentrating on it as a limited set of values.  Since I am working with a modified key block approach to constructing my plates, I have the opportunity to do a trial proof (or more than one) to explore the color set once all the plates have been cut.
For the trial proof, I mix a small quantity of all of the ink colors that I am going to use and then print each plate in quick succession, wet in wet.  I dust the print with baby powder after each impression to stabilize the ink somewhat so that the following color will trap better.  I tap out each color onto a color card and record the inks that I used to mix the color (with the most plentiful color first).  When I go print the edition I use this card as a guide to mix the larger quantity of ink that I will need.  I find that having the color splotch and the list of colors involved allows me to hit the color pretty accurately.  I make the color cards by printing on scraps of editioning paper with a laser printer.
In general, I use a split primary pallet (two of each primary color, a warm and a cool shade) along with a set of secondaries.  I also use a couple of whites and tint bases.  When I am mixing colors I am concerned about the value (how light/dark) and the degree of transparency as well as the hue.  Since I often stack colors the degree of transparency makes a big difference in how the colors are perceived in the finished print.
Here are the color cards for a recent print.  These are presented in the order that they are laid down in the final print, running as usual from lightest to darkest.  This particular print has a complementary color scheme with cool colors on the extremes of the value spectrum and warm colors in the middle.  By the way, all of these are lithographic inks that I will tweak to lower the tack and viscosity before they are printed.
The first color started by mixing a dark blue. Daniel Smith Indanthrone Blue has a mass tone similar to an Ultramarine blue, but the undertone is quite a bit different.  I find that it is almost a perfect compliment to Handshy Sun Orange. I slowly add the orange to the blue to pull down the saturation of the blue (make it more grey) and make it darker.  When I get it close to where I want it, I pick up some Graphic Chemical Opaque White and add a very small amount of the mixed blue to it to create a tint of approximately the right value.  If necessary, I will tweak the tint with tiny additions of the blue or orange to adjust the hue and bring the tint down to the proper value.

Color #2 is also a light tint (actually lighter than #1, but they do not overprint in this image).  Again, I start by mixing a hot orange by adding Sun Orange to Daniel Smith Hansa Yellow Medium.  Then I mix it into the white as before.  The addition of Graphic Chemical Laketine lowers the opacity of this color somewhat.

Color #3 is a hot orange using the same Hansa Yellow Medium / Sun Orange combination as before.  This time, however, some Laketine is added to the mix, but no white.  This results in a lighter color (more transparent) but doesn’t bleed out the saturation as much as white would.

Color #4 is a very warm (almost hot) red.  Sun Orange is the base.  A very small amount of Indanthrone Blue is added to pull down the saturation.  The mix is then pushed to the red side by the addition of a small amount of Handshy Burgundy Red.  This is a cool, very transparent red, with an extremely high tinting strength.  The result is a bright, fairly transparent color that is used full strength (no white or Laketine)

The final color is a dark, transparent blue.  This is actually the same color as the base blue for #1.  Because it is so transparent, however, and is printed almost completely on top of #3 & #4, it reads in the final print as a very dark, very cool red.

And the final print.

Color Woodcut
21 1/4″ x 19 3/8″
Edition of 15
Image Copyright 2010, Dean Russell Thompson
All Rights Reserved