You have perhaps done nothing to cause this change in the color of your pumpkin plant. Pumpkin plants are very fast growing plants and they have their roots develop very quickly and spread over a large area. Make sure there is ground there the vines of your pumpkin plants are anchored otherwise if they start growing over stuff like concrete or runners this can choke off their access to nutrients and water and can cause it to weaken, wilt or even die.
