I need a product module with options and auto SKU

I want to have a product with different size and color options. Each combination has it’s own SKU and price, how / what ext. can I use to give me this ability.

Example:
I want to sell a phone:
I want one product for the “ES Phone”
But I have 2 sizes and 2 colors… so I have 4 SKUs
White and Black, and 16gb and 32gbs

So the Skus should be:
ESPHBLK16
ESPHBLK32
ESPHWHT16
ESPHWHT32

I want the customer to be able to have a drop down for size and color and have the sku be put together based on the 2 options chosen.
HOW?

THANKS