This problem just started a few days ago. The mixer brush was doing its job by mixing colors; but all of a sudden when I tried to mix blue and yellow to create green the result was gray. I tried doing in with CS6 and CC2017: same result. Don't seem to have a problem with any other colors. This is really strange because when i was creating the demo for my class, it was working just fine. Now all of a sudden, I'm unable to mix yellow and blue to create green. Any ideas? Is there a preference that I have to change? I reset all my tools and preferences in photoshop. Thank you.