could be lack of feed. give them a dose of epsom salts a teaspoon in a gallon of water will help them green up. of course we havent had much sun either.

Is it the actual fruits going yellow, or the leaves ?
Some varieties of courgettes are supposed to be yellow.
But if it's the leaves you're asking about, Seaburngirl's advice will help.

needs feeding definately

I've got yellow courgettes growing and they are normal!