Certified GKH Gardening Expert
The following articles should be of some help to you: https://www.gardeningknowhow.com/edible/vegetables/zucchini/solutions-for-common-problems-growing-zucchini.htm
This can be caused by spider mites, which can suck the juices from plants, leaving them pale in color. Treat with insecticidal soap or see article above. It could also be due to some type of bacterial infection. Remove infected leaves/plants.