So, one side is nice green with white dots as usual, but the other side (the side on the less blurry part of the picture) is yellow instead of green. Is this some disease? I've gotten normal looking zucchinis out of this plant before.
I can't say definitively, but if it's growing just fine then I would let it grow to maturity and save the seed to see if the mutation is stable on its offspring. You could have a nice mutation for a new variety on your hands!
Yellow fruit is a normal gene on squashes in general (zucchini is a squash). It could be that something in the environment caused a recessive yellow gene to trigger on the part facing the sun (it's usually the other way around!) There are some yellow-green, bicolor zucchinis that are really pretty. If you're lucky, this could be the start of a new heirloom.
What does the transition between the two colors look like?