Keeping Records on the Yarn You Have
Telling the difference between cotton and cottolin
Cottolin has a black sharpie mark on the edge of the tube
Keeping Track
I recommend cutting off a reeling of the yarn, and taping it into a notebook with the color number.
If you want to get fancy, you could add the date you received the yarn and the projects you used it for. I have a hard time keeping records and this is the limit of what I can do. It makes a big difference in matching colors later.