Thats a good question, I'd like to know also, I've done the same thing with other plants and regretted that I didnt paint it first. I'll be looking forward to the replies also. FWIW, I've used acrylic paints and always sprayed with a sealer on the outside.
Christine

Thanks Christine

You could use a paint used for concrete, masonry or stucco. The problem is making sure the outside of pot is clean first then allowing it to dry out thoroughly which means your lemon needs to go without water for a few days.
The terms latex and acrylic are the same paint. Stucco elastomeric paint usually has a better vapor permeance (perm) rating so that it can breathe better than other paints which means it shouldn't bubble up. Some of the more expensive paints at big box stores and paint stores will adhere to just about anything and may be a good choice as well. You could also use concrete stain products but the color selection is more limited because of the terra cotta color combination. If you were going for brightly colored pots you could use a high quality exterior bonding primer as the base coat then use most any quality exterior paint over that. You may want to troll your local Lowes and Home Depots for there oops paint which are usually marked down 75% anywhere from $5 to $10. Some stores only mark down 10-25%. You might be able to find a good stucco/masonry paint as a base coat and then use your choice of exterior color regular paint. It's best to try a small sample test on another unused pot or at least an inconspicuous area on the pot that you want to paint.
Best of creative success!
Tony

I think you just inspired me to try some transparent concrete stain on some of my terra cotta pots to see if I can make a more earthy brown color.

I would first try painting a different pot of similar size .... then swap pots with the planted one and then do that one when it's cleaned up. :)

Thanks Tony...

I think the idea of painting a clay pot is fine. The dynamics of the pot may change a bit. Clay pots "breathe", and thus require more water than plastic pots. Paint will somewhat seal the "pores" of the pot. Not a deal-breaker...but a consideration.