DMC (Dollfus-Mieg and Company) is a French company that got started in the year 1746. They produce, among other things, embroidery floss and yarn for crochet and knitting. It is the most recommended and used thread in the world. You wonder what it has to do with Diamond Painting? When making the drills they use the same color code as the one used for embroidery floss (DMC-number). As a result you can combine the left over drills with other drills from other paintings. However they can have some different variations in color. Since there is different companies that produce them. Which make it important that you check so that they have the same color before mixing them.
You have 454 different colors that is being used for Diamond Painting. When you get a kit the bags are often marked with the DMC-number, or you have a symbol chart that shows what the DMC-number is. Black for example have DMC-number 310. For white they usually use 3865 or B5200. Some premade kits only have a serial number and not a DMC-number. In those cases you can save the drills and then compare them with the ones you have DMC-numbers for. In that way you can quite easy find out what colors you have.
For the embroidery tread DMC has they have created a color chart so that you can se what color the different numbers have. If you have tread that you don’t know what number it has you can compare it to the color chart and found out. This is something you can do for Diamond Painting as well. Altough the drills can have different shades you can make your own color chart so that you know what number corresponds to which color.