How do I get an uneven edge to the yellow paint on the columns and wall. I want to fade it or roughen it somehow. Having the yellow end at the exact same line everywhere looks too artificial to me.

There are several ways to do this.

You can insert a Noise Texture and a Color MixRGB between the Mapping and Gradient Texture Node:

This is a common technique that is used the most, I think.

If you want to fade the transition, then change the interpolation mode from the Colorramp.

Perfect, thx - i was trying to figure it out but I couldn't get my head around it :-)