Camilla's coat looks like patchwork to me. Somebody has done some very careful fitting I think. If that was mine I'd have to make sure I selected the right piece of underwear to go with it.
I can only speak from a dressmaking point of view, but I think you'd need to decide whether to design the garment with a "pattern repeat" - for example matching pieces of fabric on both sides of the front and across the back, or, to make a piece of patchwork from evenly balanced shades in a completely random way (I'm getting out of my depth here!) and just take it as it comes. I think maybe you'd have to match squares (or whatever) but not necessarily patterns.
Edit: What
@realale said!