Yes, we do print on colored papers and envelopes. See the papers and envelopes we stock. Some customers supply colored paper and envelopes which we can print on and we can special order some papers and envelopes.
There are some things you should avoid. For instance, light ink letterpressed over dark paper does not work well; dark ink over light colored paper works better. We now offer white digital addressing and return addresses on envelopes that are too bright or dark to accept letterpress inks. Foil stamping is also a good option for reply envelope addresses and return addresses.