Korny Klown2 But I don't understand why the color map slot has no “Choose Operator” dialog because this seems to be an obvious feature at this place, isn't it?
That's a good point, as it would be handy.
The explanation is that it appears the parameters that are exposed to the user does not have this capability. You can see the available options by going inside the shader, finding the bit that exposes the parameter field on the UI. Play with the different options. It needs to be a string for the shader to work.