To answer a color set query that does not specify exact match, a color set distance back-off is used. The idea is to return first the zero distance items 
then the 
items, etc., until 
images are found. For the matches the specified color set is used verbatim. For the matches the group of color sets that are within distance one from the query color set are found. Then each of these color sets is looked-up in the index and the matches are read off. This is repeated for the 
matches, etc., until images are retrieved. By using this method we avoid an exhaustive search of all regions in the database for each specified region in the query.
When several color regions are specified in the query the individual color region distances are summed. This computes the overall query color distance value 
for each image match using 
where K is the number of regions in the query. The current implementation requires that matches are found for all specified regions in the query in order for an image to be considered as a match.