I am trying to create a solution where I can create a series of ‘designs’ or ‘patterns’ which may use 3 colours from a palette. What I want to achieve is to be able to change those colours (my palette) and have them apply across all of the different designs in my document.
Below is a very crude and gross diagram of what I’m trying to achieve. Can this be done in Photoshop or Illustrator?
You can do this in Illustrator, but not in Photoshop; see https://graphicdesign.stackexchange.com/a/13792/57377.
To do it in Illustrator, in the “swatches” panel, click on a swatch and then click “swatch options” in the hamburger menu of the swatches panel. Turn on the “global” checkbox.
Now, anything that uses the “global” swatch will update when you edit that swatch.
Note that your existing objects that are colored the same CMYK value as your global swatch will NOT update. You have to actually color them with the global swatch first.
Global swatches are indicated by a small white triangle on the lower right corner of the swatch square in the swatches panel.
Here is the official help for illustrator swatches.