I've grown the before, mine stayed purple for a bit, then turned green, then red. They'll get a little bigger in my garden than that before losing their color.
I'm not a HUGE fan of green peppers, so I stopped growing the ones that don't ripen to color. Red is my favorite, and then yellow is my husbands favorite! 3 years ago, I grew 32 varieties of sweet and hot peppers. ( My local greenhouse is very friendly, and gave me the plants as long as he could come take pictures. He's also a pepper and tomato enthusiast! Super awesome business, too. Stays organic. If you want I'll pm you the link.) It was fun seeing all the sweet pepper varieties! My favorites were the white bells! They were very neat! I learned a lot that season about how they grow, and what does well in my climate!
USDA Zone 7b/ Sunset Zone 31