Google has added a handy color filter to its image search. Simply conduct the search and then look for the color box to filter down the search.
Interestingly, you get different results if you actually type the color in with your search. The screenshots below show:
1. a general search for "bike"
2. a general search for "bike" with color filter set to "green"
3. a search for "green bike" with color filter set to "Show all colors"
(Also, color filter didn't show up for a search for "ice cream")