Throughout the construction of the Mishkan (Tabernacle) and Bigdei Kodesh (holy clothing for the Kohanim), they use a mix of red, blue, and purple wool (plus [white?] linen, and in the case of the clothing, gold thread as well).
I'm sure there are several easy and obvious answers here, but for the sake of acknowledging the question (which I'm sure some kids must have asked):
If you're already mixing red and blue thread, then why the purple thread? Wouldn't it look purplish anyhow?