Google is said to be doing more experiments with the colors of the stars in their search engine. Usually we see orange colored stars next the links of sites that are making use of rich snippets but it appears that Google is currently testing different colors.
A Google spokes person has told searchengineland that they are indeed experimenting with colors and we may see more experiments along these lines going forward.
We have been seeing a mix of green and red stars in the search results this week. It would be great to know the official reason of why Google are wanting to change the color of the stars. The green looks nice but it does make it blend in, where as the red had the opposite effect.