Not enough zucchini, substitutions?

Yellow squash is interchangeable. Carrots will probably work. But it also depends on what the recipe is--a baked good? a gratin? a fritter? a filling for a pasta? May I also congratulate you on being short of zucchini--the cliched problem is an excess...

I use carrots if not enough zucchini. Next time try a combination of both, quite good.
BB💐

Yellow squash (thinking you mean crookneck squash) is perfect, it will have a very similar water content, flavor and cooking time.

Yellow squash is so close a substitute that it barely counts as one.

Half cup won't make much difference in most recipes, but some impact if it's a cake or pastry. Yes, I would include some carrot or squash, which will give flavor, color, volume & water.