Choose a useful ground
Use a small piece of the cloth you regularly stitch, or make separate sections for dark, light and natural grounds. Thread can look radically different against indigo and white.
Leave room for a few stitch lengths, a short line and a knot or join if those details matter to your project. A single loose strand is not enough evidence.
Record more than the colour
Write the brand, shade number or name, thread range, approximate weight and date. Add whether the sample used one strand, a full sashiko thread or a divisible bundle.
Note the needle and stitch length when you are comparing close options. Those details explain why two samples of the same colour may look different.
Store the card with the thread
Punch a hole or use a ring so cards stay together, then keep the active project samples with their matching skeins. Protect the card from direct light so the reference remains reliable.
Add a small washed sample for colours you use often. The post-wash appearance is more useful than an untested promise when planning a garment or repair.