It's a very good test to do, both with the text and without even. You're covering the 3 E's (Eyes, Emotion, Excitement), you're showing the end result (the brownies), the purple color is good but "cyan|lime" not as popular (but this depends on your competition too. I'm going just off straight averages of numbers). That's about it, I don't see any problems with further analysis. If I did think there would be I would submit the image to (Google Cloud Vision) and have it analyze it further to get it's opinion "which is very important" on a questionable thumbnail. I would have provide a link but external links are not allowed but a simple Google search should pull it up for you.
Hope that's helps.