I have posted a bit of a rant about this subject here. It's the naming of the humbroll paints. They have to colors called "olive drab". 66 and 155. I have them both. 66 is olive grey while 155 is the classic OD.
To make things even more harder to figure out, there are errors in the paint chart as well (at least in my copy, I don't if they have fixed it already) Take a look the paint names in different languages. In german the two paints have different names. "Olivegrün" (olive green) for 66 and "olivegau" (olive grey) for 155.
(and with vallejo, OD is called "brown violet" while their "olive drab" is actually a much darker color)