Years ago I petitioned all my friends to give me their used bus transfers.
I used them to create swatches that would show me colour variations from brand to brand in the paint I was using. I also found that the swatches were a good way to record the transparency and sheen of each colour.
All you need to do is gesso over the blue text and you are ready to start painting swatches and label them with pigment contents. Which is great for when you want to recreate a colour that you have mixed yourself out of a variety of other colours.
Vancouver's bus passes have the most perfect thick black bar running down the side which is great for showing how transparent or opaque a colour can be. Below are a few swatches from my collection:
