Foliage reflects IR & the purpose of IRR paints is to give a similar degree & pattern of IR reflectance as would be found in NW European woodland. If the paint was absorbent then there would be a vehicle shaped IR "shadow" that would draw attention against the IR refelective foliage.
The quote was from Army Code No.60503(PAM 3), Materiel Regulations for the Army, Volume 2, Vehicles & Technical Equipment, Pamphlet No.3, Painting of Army Vehicles, Aircraft & Equipment, Annex B, Dec 1980
Ian what you said may well be the case in the field. What is written in Regulations doesn't always get carried through! I have no service experience & go only go by what is printed. So input from the front line users is always welcome:)