I was running into this same issue and it is because the Web Browser plugin is outputting its color data in the sRGB format and the Widget/3D Widget is expecting the data in linear RGB. The problem can be fixed by creating a new material to use in the 3D Widget instead of the default Widget3DPassthrough_Masked_OneSided. I have included a screenshot of the material I made to fix the problem. It implements the standard sRGB to linear formula.
