I tried searching for "pink horse" and "horse" with pink color option and got slightly different result sets. It seems like the first search returns anything related to "pink horse", including titles and captions. Whereas the second option seems like it's searching for "horse", then filter that out and returning only pink images.

ps I don't see the "All Colors" drop down in my blue bar. But you can do the same search by appending "&imgcolor=yellow" to the url.

[edit] It didn't show up on Firefox, but showed up fine when I used Chrome.

But you're relying on people to use the word "red" in the text accompanying the picture.