I started using a expandable area for the first time in UMG, and i don’t know why, but the border brush color is dark grey for absolutely no reason. It should be white because it automatically will darken any color you try and set it to.
The fix is simple but may not be very straight forward for everyone, all you have to do is make a UI material and put 1 (white) into the base color. Then use this material as the image for the border brush.
Its my opinion that you shouldn’t have to go through this extra hoop and it should just be white by default, like every other widget i have used previously.