Bug: Color used in Feedback Master changes to a different color in preview

I'm experiencing a bug in Storyline 3. As shown in the attached images, I created a black background box in the feedback master, and it shows up as a black background box in the feedback layer on the quiz slide. However, when I preview the slide, the box inexplicably changes to blue! 

The black box is just a simple box, no states or triggers or variables attached or affecting it. I tried deleting it and creating a new black box, but same problem. I saved the file, closed it, reopened it, same problem. 

I also tried placing the black box directly in the feedback layer on the slide, but I can't put it behind the green feedback box because the green box is on the feedback master. The only thing that works is to get rid of the feedback master and just recreate it in the layer, but that defeats the purpose of having a feedback master in the first place.

Why is this happening, and how can I make the feedback master stop doing this to me?

Thank you!