The number of masters in your set will depend on your needs and perhaps your budget. It’s a good idea to begin by spreading them out, collecting for example an “E”, an “H”, and an “I”. This will give you comparisons for D through J colors. Remember the master is at the top of its grade category; so when comparing a master “I” to a suspected “J”, you will have to compensate slightly. If the stone is slightly lower (lighter in color) than the master, then it is probably a solid “I” color.