I am not familiar with the Yellow Contender. The regular Contender is green. There are several possibilities. 1. Many of the yellow zucchini will have varying amounts of green on them, which varies with ambient heat. The round yellow One Ball for example starts off a bright yellow and by the time temps get to triple digits, turn completely medium green. 2. Cucumber mosaic which causes mottled greening in yellow squash ( Your pics don't look like Cucumber Mosaic)3. Impure seed, cross pollination only shows up in the next generation.
There are lots of multicolored summer squash Even more winter types.

Thanks farmer..