Is it a way to change the color of two materials at the same time with the color attribute ?
As well I would like to be able to change the thumbnail preview of the color. Define the color value and the thumbnail independently.
This is not possible with the colour attribute.
What you could do is to put the objects you want to change in a material group.
Then both would change at the same time, and also you can give materials inside a group a different thumbnail if you click the little pencil next to the material.