To warm up, try combining any two colors together so that neither one dominates the mixture. Make a patch of the result on a sheet of student grade paper.
Next, mix two complimentary colors together to make a neutral with neither of the components dominant.
Finally, mix three colors together to make a color where none of the three dominates.
OK, now that you're all warmed up, look through magazines (do people still have magazines around?) for patches of solid color at least 1x1" square, and cut them out. Paste them on a your piece of watercolor paper.
Using only one red, one yellow and one blue, make as good a match as you can for the colors. Make a patch of the color next to the cut out original. Write the names of your primaries beside the patch.
Now switch to a different set of primaries, and match the colors again.
That's it. Have fun.