If your product has multiple color variants, you will also need to submit different images for each variant. To do so, an image will need to be added to a "Color" product feature (“Color” here is an example of product feature).
Go to : Store » Products » Product Features. Then click edit
the “Color” feature.
You should now see all of the features in the category. Click the "See All Feature Fields Displayed" to view the page in the full viewing mode.
Now, scroll to the right until you see the "Product
Variant Image" section. You can then upload an image to this feature by
clicking the Upload link.
Google recommends the variant image to be at least 250 x 250 in resolution so you will need to make sure your images meet this requirement.
If for some reason you are not able to provide a different image for each variant, you can try to apply for an exemption from Google by submitting a request from :
http://www.google.com/support/merchants/bin/request.py?contact_type=item_error