Access 2000: Compare Fields

I have a access database with a Order form. On the form I have the following: Style, Color, S, M, L, XL, XXL. Style & color are pull down, qty is entered into the others. When an entry is made into one of the size fields, I would like it to compare that number to the actual colorinventory(table name: colorinventory) number to determine if the size will be backordered, then disply a message stating that a backorder will happen.
Thanks for the help.