Google Blog tells you how to do this:-
Just search for something the way you normally would, such as [tulips]. On the results page, click on the "All colors" drop down in the blue bar and choose a color. For example, try restricting your results to [yellow tulips]. Want to see purple tulips instead? Simply click on the color filter again, select purple, and voila — you have pages of beautiful images!
You can also combine the color filter with any other image filter to further refine your search. For example, if you're looking for an image of an orange butterfly, try restricting to photos or clip art.
Color is one of the basic visual elements of an image. Whether you're browsing through photos of sea creatures and flowers or searching for the perfect orange butterfly, we hope you like our new color filter. We've been rolling this out gradually, but it will be available to everyone soon.
Till the time Google rolls out this feature to everyone you can use it manualy by adding <&imgcolor=red> to the result page address..
E.g to search for a bird with color red use this:-
(you can replace "red" with "blue", "green", "teal", "purple", "yellow", "orange", "pink", "white", "gray", "black" and "brown")