What are the color descriptions?
select color_description from ref_colors
Which ones are used only once across all the products?
select t2.color_description from products as t1 join ref_colors as t2 on t1.color_code = t2.color_code group by t2.color_description having count(*) = 1
How about for the characteristic names?
select t3.characteristic_name from products as t1 join product_characteristics as t2 on t1.product_id = t2.product_id join characteristics as t3 on t2.characteristic_id = t3.characteristic_id group by t3.characteristic_name having count(*) = 1
Show the ones that are used at least twice.
select t3.characteristic_name from products as t1 join product_characteristics as t2 on t1.product_id = t2.product_id join characteristics as t3 on t2.characteristic_id = t3.characteristic_id group by t3.characteristic_name having count(*) >= 2