Can I manage stock per option (color / size) for store products?

Yes. When you add options to a product — for example Color (3) × Size (4) — Diffsome creates a variant for each combination and tracks stock separately for each one.

So you can set White / M to 10 in stock and Black / L to 0. Out-of-stock variants are automatically shown as sold out and can't be added to the cart, while the rest stay available.