I have over the years made up 3" x 5" colour cards, with three (captioned by maker/their colour description) swatches of paint on each. With over a hundred swatches now on hand, I can match existing paint on models when changing out parts like rooves, or commercial paints to prototype photos.
If a paint is discontinued, I also have reference for matching or approximating it with a currently available paint.
I am approaching "fifty shades of" FC Red on those swatches...
