Here's one way, but I'm not sure that it's the best. Create a table called ForbiddenPatterns with columns Size, Colour, Pattern. Insert one row for each forbidden combination. Write a function that takes Colour, Size and Pattern as parameters and returns Forbidden if the combination supplied appears in ForbiddenPatterns. Add a check constraint to your original table that checks that the result of the function is not Forbidden.

John

Edit - corrected typo