foreign key help

I am building a database that has about 20 fields in various tables that
I would like to constrain to a small group (<=5) of choices. Rather
that refer to 20 seperate tables, I would like to refer to 1 table that
contained category and choice fields.

for example

choices table

category | choice
-----------------
color | red
color | green
size | small
size | big
shape | circle
shape | square

item table

item | shape
-----------------
box | constrain to choices(choice) where category='shape'

Is there an easy way to do this, or is it better to have many
choices_category tables?

Thanks,

Dale