Are you sure that it's a pepper plant and not a white aubergine? Peppers should start off green and turn to red.

Sometimes green peppers can get white streaks on them from bright sunshine, especially through the glass of a greenhouse. Otherwise it isn't clear what you mean. Green peppers do take a long time to start to colour up, and even here in central France our outside peppers are only just going yellow (one variety) and red. They need plenty of heat to ripen.

There is a "white" form of bell pepper (actually creamy). Maybe the seeds or plants were mislabelled?