I am looking for a way to define a set of colours globally to be used for (other) materials.
For this there is a colour palette node (paid) and solved with python.
Since the API changed and I don’t know anything about Python it’s probably not an option to buy the addon or try to code in Python. I was looking for custom properties and drivers, but that is not very convenient.
I hoped there was a way to share the settings of a node-group with other materials. This seems no to be the case.
Node groups are able to do what you want. The trick is to not put in an input. See my image, where the RGB node (for the colour green) is put into the group output. Now, all materials that use this node group will have green. This can easily be done with other colours, and for ease of differentiating between them when adding them, use names. This can be set in the properties menu (N).
EDIT: I think I may not have answered your question. If you want to have the input setting to be uniform across materials, you may have to disconnect them from the input part of the node group, and only change values while inside the node group editor panel.
If you use a colour scheme designer, I suggest plugging in all those colors to the outputs, perhaps labelling the outputs. Sadly, replacing the node with a stored alternative requires manual labour or some script that I don’t have. I don’t have the artistic freedom to do as strong schemes as I’d like to.
Aha @unyxium and @CarlG
I will experiment with “adjusting has to be done inside the nodegroup” in order to get things done globally across multiple materials.