Putting this up now in case anyone else finds they have the same problem.
I had a similar issue to the OP, and not just with the PNG format, if I imported a Photoshop PSD the same thing happened; faded looking image when displayed in a widget, even though i set the Image Compression Settings to UserInterface2D, and the Texture Group to UI.
It seems to be an sRGB related issue, what worked for me is:
Export from Photoshop and DISABLE the sRGB checkbox
Import to Unreal, apply the settings mentioned above plus, CHECK the sRGB tickbox.
Then the colours appear correctly in widgets.
sRGB OFF in Photoshop, ON in Unreal.