What is the description of the color used by least 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 ORDER BY count(*) ASC LIMIT 1
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 ORDER BY count(*) ASC LIMIT 1
[{'color_description': 'black'}]