The two colors idea is a good one -- with two differently colored practice cords, you can see what is going on better (for many knots).

Two different diameters are also good to have. Some knots, like the Prusik, are meant to be used with one larger and one smaller diameter.

I went with 2mm and 3mm, which is working well.

(You could also just get a couple of extra strands to practice with. They are typically just 10-20 cents a foot.)