Can you hybridize, let's say; Bell Pepper with ornamental variegated pepper like 'purple flash'?

Since they are both Capsicum anuum; I would say 'yes'.

I purchased 'California wonder' bell pepper to use in the experiment with ornamental pepper. But is green the true color of the pepper or is it red mature?

Red. I don't grow the Bell Peppers but they will be red when fully ripe. For a sweet pepper I grow Cubanelle; here they are ready to eat as green peppers, and fully ripe as red peppers. It's the same with Bell Peppers.

Just to clarify -- not all bell peppers will be red when ripe; some are yellow, some red, some orange, some purple... but Cal Wonder is red when ripe, when it's green it's -- well -- green (as in, immature). (some are also whitish/cream colored when immature) They are definitely sweeter when ripe, although that was not the question.