squash and zuchinni

Squash and zucchini are very heavy feeders... try magnesium sulphate..(Epsom salts) mix a tablespoon to a gallon of water. makes sure the soil around the roots is moist or newly watered then dissolve the salts in a gal of water and apply...should green up fairly quickly...
It may not be a deficiency, if you have recently moved them from indoors or from a greenhouse into the open garden they may be shocked..give them time to recover...keep them well watered and in about 14 days they will have adjusted (hopefully)