As explained by David Doyle, I'm still learning too. I'll find the reference in a minute...
Here:
"The FS595B 24084 should be darn near a perfect match for FS595A 24087, and FS595B 24084 is available from a number of commercial paint makers. 24087 is a radically different color between FS595A and FS595B.
The problem that many enthusiasts experience, especially those new to the hobby, is that when the classic M-series vehicles were built, the Federal Standard in place was 595A - but today the Federal Standard in use by the military, industry, and hence the paint companies, is 595B. Almost all colors are the same in both number and colors between the two standards - BUT our beloved semigloss OD is the exception to the rule. The number 24087 appears in both standards, but the shade changed rather dramatically.
HTH,
David Doyle"