Usually those things are customer specs on contracts. Iran and Saudi had the brown furniture. Many times they are trying to make the firearms lower profile in a tactical environment. In a desert environment like Saudi Arabia and Iraq you could see a vehicle 10 km out or more. If it was approaching you it might take 15 minutes for it to get close enough to identify what type vehicle it is. Difficult to be "camouflaged" but it helps trick the eye so the long profile of a big black rifle doesnt immediately catch your eye. (Wet helmets gave away a lot of GIs in the Pacific theater in WW2, the shape of the helmet with the sheen gave them away)That helps you get closer to contact before being identified as threat, or lowers chance of them identifying you quickly as they enter your AO. The tropical color was meant as a selling point for jungle like environments where many of the leaves can have that darker green and get a sheen from moisture or the type of leaf. The double and triple canopy jungles of sections of Panama meant most contact was going to be close range and everyone wants that extra edge hoping it will help. Its also the reason special ops people love rattlecanning their firearms. it lets them more closely match their specific environments. Regular soldiers are not given that freedom, uniformity, fear of soldiers damaging their weapons, etc. (Our Airborne battalion had shiny m16a1s from them scrubbing the "Carbon" off, fortunately when the M16A2 were issued the tired shiny ones went away)
Just some thoughts on it.