in your case, each product is comprised of
your inventory table would need to take each combination into account:
stockID - primary key
stockProductID - foreign key relation to the product
stockSizeID - foreign key relation to the size option
stockColorID - foreign key relation to the color option
stockStyleID - foreign key relation to the style option.
stockQTY - quantity of item in stock.
eCart will automatically set up code to update the inventory when the transaction goes through, you would need to hand code this logic.