Green seems to have become the convention of the lithograph printers during and after the war. Indeed, the common delusion that the US wore uniforms from 1812 was evidently propagated by the same lithographers!
It seems improper, somehow, for me to tout the new and expanded edition of "The Mexican Soldier" on someone's else's thread, but since it's been brought up by others, I'll take that as an invitation to at least provide a direct link
Regarding Anton Adams "The War In Mexico," I cannot more highly recommend it as a single source on the war. The OB's alone are worth the price of the book.
However, due to I think rather inadequate support for Tony's work by his Publishers, you'll find that the flag information is not only spotty, but often dead wrong. One can see where the "artist" they found for that part of the work has simply copied the wrong the flags from the source he was working from.
Similarly, the rather puerile uniform plates are simply point for point copies from Hefter. Not necessarily
inaccurate, but definitely a secondary transcription. Whenever the primary source is available, why go anywhere else?
The Rene Chartrand title by Osprey is another "must have" and is the closest thing to a true successor to Hefter's work.
TVAG also has the best resourced and finely rendered flags for the US and Mexicans both anywhere. No brag, just fact. That we have been plagiarized is a left-handed compliment. These complete sets are available in most every game scale here: link
And, easily the most comprehensively researched (some 9 years) rules on the subject, "Gone To See The Elephant," may be looked at here: link These rules can't be everyone's cup of tequila, but there's information provided and built into the game that just doesn't appear in other sets.
If 15mm is your thing, we also have the 15mm Special Mexican Army Range of ancillary figures which serve in GTSTE, and are otherwise unavailable in any other scales: link
Finally, a comprehensive--and FREE--resource (borrowing heavily, but not exclusively, from the French language Website alluded to, above, I suspect) for US Uniforms and weapons may be consulted here: link
And, no, I wasn't asleep--YET!--just busy organizing the first restock of the "Viva Juarez!" 25mm range of figures for the French Intervention that should be ready for general sale again starting in January.
I can't seem to get north of the border these days!
TVAG