When building a 'client-representative-user-interface', you usually need to implement elements everyone understands. A client can get a rough understanding by playing around with knobs, buttons, sliders, etc. But it's perhaps even easier to implement certain elements where people are already familiar with.
Herewith, a simple 'colorpicker'. Most photo and image editing software are using a colorpicker in a equivalent way. I personally liked the picker form Serif Affinity Designer and rebuild it from the ground up in Touchdesigner.
Play with it! Let me know how this works out and always feel free to comment for any suggestions/additional features, bugs etc.