Primer sealer.

[OldDude]

Different coloured nail polish will work, but I just use a Sharpie.

For load development, I use different colored Sharpies on the primer. This works best with nickel-plated (CCI or Federal; maybe others) primers.
The load identification I was thinking of is to distinguish between different types of bullets that otherwise look similar.
The visual differences between a Sierre 155 and a 168 MatchKing bullet once it's seated are not that obvious.

 

[Bubba Yugga]

For load development, I use different colored Sharpies on the primer. This works best with nickel-plated (CCI or Federal; maybe others) primers.
The load identification I was thinking of is to distinguish between different types of bullets that otherwise look similar.
The visual differences between a Sierre 155 and a 168 MatchKing bullet once it's seated are not that obvious.

I just mark over the headstamp.
Once you run out of colours, you can do single dot, two dots side by side, two across from each other, slashes, etc.

 

[270WSMXBolt]

Just sharing content from a contractor. Clearly a different level than the home kit.

 

[fred0000]

I used to use a markal paint pen, but generally only when a primer went in too easy, i'd seal it in, and it would also be my indicator to retire that piece of brass.