Basically the sku is a unique identifier for you. If you want to have the sku be different for different options so that its easier for you to inventory your items or get the correct product to the buyer, then you should have different skus for each option. Its up to you on what works best for you for sku. If you provide a little more information about your store, I might be able to give you some pros and cons on how to set up your skus if you're still unsure on how you want it set up.