Yellow zucchini develops a golden color as it grows.

Yellow zucchini is a variety of summer squash, just like regular zucchini. The main difference between green and yellow zucchini is the color of the skin. The golden color develops as the fruit grows. This warm season annual needs frost-free weather to grow properly. Growing yellow zucchini provides the home gardener an inexpensive source of summer squash.

I wouldn't let the squash get large and I would remove the larger yellow ones now.  If you want to eat them you can.  It would be better if you picked the squash when it is smaller and not wait for it to turn green.  I really don't have an answer for your question.  Other than getting the wrong seeds have you fertilized the plants?  Good luck and enjoy.