You make a good point about the blending in watercolor but can you try something for me....
Put down the yellow
Then put down the blue
Then, can you use your eyedropper (color selector) tool in the center of where the colors overlap and see what color comes up?
I was just looking at this with a co-worker after reading your post and although the color didn't look green, when we chose it with the eyedropper tool, it was most definitely green.
Can you try that and let me know what you discover? (Either way, thanks for the feedback, I'll definitely share it with the Sketch team.)
Definitely not green on yours. Thanks for checking. I'll share your comments and your screenshot with the team.