Chlorosis can make leaves that are supposed to be dark green, light green. If it's bad they'll turn yellow. No one reason for chlorosis. Can be a nutrient deficiency, root damage, or poor drainage.

Need a picture for an improved chance of a diagnosis.

There are 2 big leaves like this.

Wow, you really mean bicolor with a sharp boundary. Not like any chlorosis I've seen. There are squash that are yellow and green, with a sharp color boundary (e.g. Zephyr), but I've never seen leaves like that.

The sharp green-yellow color boundary makes it look like some corrosive liquid spilled on the leaf, except I gather there are no similarly weird leaves underneath it where the stuff would have dripped off. I assume this isn't some yellow coating on a green leaf like you might get from a prankster with some paint.

Looks like a chimera, a genetic mutation. This happens frequently in nature, especially in the plant world.

Edited to add that horticultural chimeras (ky-ME-ruh) are extremely important in the commercial horticulture industry.....some of our most popular variegated plants are cultivated from chimeras discovered by accident and asexually propagated forevermore. ;-)

There are only 2 leaves like this on the plant. It is a huge plant, vining all over the place. Trying to take over my squash patch. Nothing has been spilled on the leaves. It isn't a yellow coating. The leaf is definately 1/2 yellow and 1/2 green. It hasn't changed any in the last week.

The leaf mutation won't cause your plant any harm but you can remove them if their appearance disturbs you.

I kind of find it interesting. I'm waiting to see if anymore appear. Thanks for your help!