So, are all the elements you have 8bit pngs? (Character too?)
Couple things I’ll mention that worked for me.
Compression Settings: UserInterface2D(RGBA)
sRGB: off
Alpha Coverage Thresholds: 0,0,0,1
At the top of each texture’s details, the Format should read B8G8R8A8
I had this issue awhile ago so there may be other problems with more recent engine versions.
You can try googling for “washed out UMG” and there are plenty of possible answer out there in addition to what I’ve suggested.
FWIW, I did read about someone who was using 8-png and getting this issue, but they imported it as 16-bit and then back to 8-bit and it went away, so some brute force may be needed. Good luck!