The Film Look made this 2-minute video showing how they created customized lens caps to help better identify and keep track of them.
After buying some cheap generic lens caps from eBay at the correct lens diameters, they disassembled them, masked the portions that contact one another (or else the paint will ruin the sliding action), applied primer, and then sprayed paint.
They then made identifying labels using a embossing label maker and attached strips of Velcro as well. The Velcro makes it so they can stick a lens cap on their camera bag when the lens is in use instead of misplacing it.
The labels make it easy to identify the lenses when they’re being stored in a compartmentalized camera bag with only the lens caps showing.
If you’d like a similar labeled lens system but don’t want to go the do-it-yourself route, a number of commercial products have emerged over the years. These include the LenzBuddy and BlackRapid’s LensBling.
Image credits: Video and still frames by The Film Look