Stock Quantity for Different Sizes and Colours

If I have one product listed with different sizes, eg. A dress that is available in Blue, Green and Red, and each colour has a different size i.e. Small, Medium and Large, how can I set stock level for each attribute, i.e. 1 x Blue Small, 1 x Green Small, 2 x Red Large.

I can only see that it has total stock count, so if I have 2 x stock, somebody may order 2 x Blue Small when I only have 1. How do I manage stock levels in this instance?

Thanks