On some of Panasonic’s coin cell batteries, you can find an insulation around the edge of the battery. The color of that insulation can differ depending on the battery. The reason for the different colors of insulation, are the different temperature ratings of the batteries. Some have a higher temperature ratings and other have a lesser temperature ratings. For example, P299-ND is a standard temperature rated battery, and P023-ND is a high temperature rated battery. As you can see in the images below, the colors of the insulation on each battery is different.
P299-ND
P023-ND
For information on these battery products part numbers, here is a topic on Panasonic Coin Cell Battery Suffixes.