I filed an issue about this a while ago. I even went as far as submitting a pull request to ‘fix’ it.
However, as @mramato pointed out at the time, it wasn’t something that is so easily fixed. Javascript allows you (and the cesium developers) to do things that are hard to codify in typescript. It would be nice to see MaterialProperty | Color | string as the type for these properties, but I don’t expect they’ll get round to fixing this anytime soon.
You can work around the issue by explicitly creating a MaterialProperty with the color or texture you want to apply.