Colors are washed out in Unreal, but not in the Rive editor.
This seems to be a color space issue for some users, but it's not correctable by toggling the sRGB option on the texture. We need to look into this.
I'm getting this issue as well.
Colors inside of Rive, in my example here to the left #730000, import as hex linear. You can see that as the bright red to the right of the image. That is the imported Rive file.
Overlaying a border (highlighted) set to hex linear 2C0000 matches sRGB 730000 and it displays as expected.
This is using a Rive Widget inside Unreal 5.3.2 with Rive version 0.3.7a-gh. This behavior was not seen in Rive 0.1.17 and Unreal 5.3.2
This is definitely an issue. We're refactoring the Unreal plugin and rendering right now. I suspect this will be fixed as part of the refactor. If not, then soon after.