Why won't my peppers go red, or orange?

It's all in the title... I have romanos that should be red

and bells that should be red or orange

... they look spectacular, but are staying green. They are sharing the greenhouse with red tomatoes and red chillies, so there must be enough sun to ripen them.

I have picked a couple and used them green but I want red and orange ones, especially as I intend to save some of the seeds to sow next year; to get the best chance of a good crop they need to be from ripe specimens.