Good idea, but the results remind me of a Target commercial:P
If you switch the query to "dog bullseye" with imgcolor=red, 10 out of the first 20 results show the Target dog. If you remove the color restriction, only 5/10 feature the dog. So this feature potentially aids search accuracy.
Keyword based searching definitely has limitations. If a search keyword does not appear in the context of any "good" documents you're searching for, the search engine will not return any "good" results. In the example above, a search for "bull terrier" and imgcolor=red only returned 1/20 "good" result. And "bull terrier" has high semantic relevance. This is because the authors of the "good" documents I was looking for (e.g. the Target dog) rarely use "bull terrier" in the document context.
Plus keyword based search engines can be easily hacked to associate garbage with certain keywords.
If you switch the query to "dog bullseye" with imgcolor=red, 10 out of the first 20 results show the Target dog. If you remove the color restriction, only 5/10 feature the dog. So this feature potentially aids search accuracy.