Let's say I have blue widgets and red widgets in two different tables. Then I have a Sold table. How should I set the Sold table up so that I have the BlueID's and RedID's in the table. Two different columns? One column that could contain both values? Or a seperate categoryID that links to a table that describes the different widget categories?

You can have something like this

Fk_id
A field that mark is the records is from blue or red