If you haven't yet gotten your good paper, you can do this exercise on just about anything. The object is two-fold: to awaken your instinct for color mixing, and to strengthen your awareness of the role the primary colors play in creating neutrals. The first part we did in class, so repeat it, or not, depending on your degree of confidence.
Choose one red, one yellow and one blue, with an eye toward the ones that will "go both ways". Look for the yellow, for example, that will make good greens and good oranges.
Mix together two of the three colors, adjusting and fine-tuning until neither one dominates. Make a small patch of the new color on your paper.
Now add the third color to the mix, until none of the three dominate. Make a patch of the new color on the paper.
Part two:Make the darkest version you can of the three color mix, making sure no one color dominates. Just keep adding pigment to the puddle (but no more water). When the paint is so thick that it looks shiny even after it's dry, you've gone too far. Add a little water, and try a patch on the paper.
Look in magazines for pictures with areas of solid color about 1 1/2" square, or bigger. Cut some out, paste them onto your paper, and try to mix a perfect match. Use the same limited palette you used for part one. Make patches of your attempts beside the cut-out.
Now try to match the same colors using a different set of primaries. Write down the colors you used near the patches.