It would be a tool you could buy from a shipwright, or found as a schematic and built by a builder for you to use.
It would take the characteristics from sails you already own (colors, types of design [e.g gradient, stripped, pirate, seafarer, X, etc]) together with “starter” sail designs, who would be just basic shapes until the player gets sail types. You would be able to combine up to 3 designs for the sail, and choose up to 3 colors to paint them.
Maybe, if it’s a good suggestion, getting all the sail designs and colors would reward you with a badge. (Name ideas: Artist of the Sea, Pirate Painter, Bronze Sea Artist, Boat Artist)
Oh thank god I read the title and thought it was going to ask for custom sail designs (as in you make them yourself and weren’t limited by preset design options).