subtracting stock with differing options

OK. Need to get my brain round this one and I am hoping someone else has come across this before. Someone must have.
Lets say we have a clothing product with several options. It can come in three different sizes. S M L. They also can come in different colours. Black, blue and green.
When a jacket sells out in small size black but there is a blue available in small. What happens?

Thanks guys n gals

Assuming you have your quantities set correctly and that you DON"T ALLOW CHECKOUT WITH 0 STOCK then the value won't even show up in the list of options. So say you have a list of colors RED, BLUE, GREEN and RED reached 0 stock. The list will not show RED>