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.
A search for "White Shirt" with imgcolor=red
Fun results