The tan follower is the new magazine, and will be phased in as the older green followers are phased out. The black follower is to be removed from service immediately.
How the longer legs stabilizes feeding
Basically, it uses a more robust follower and a stronger spring to improve reliability:
The Army has begun fielding the new 5.56mm 30 round Improved Magazine that delivers a significant increase in reliability for the battle-tested M16 and M4 weapons systems. Bolstering the already high reliability ratings of the M16/M4 systems, the Improved Magazine effectively reduces the risk of magazine-related stoppages by more than 50 percent compared to the older magazine variants. Identified by a tan-colored follower, over 500,000 of the improved magazines have been fielded to units in Iraq, Afghanistan and in the U.S.
“With the improved magazines, we’re taking weapons reliability up another notch,” said LTC Chris Lehner, Product Manager Individual Weapons. “By incorporating a heavier, more corrosion resistant spring, along with a new follower design that does not tilt inside the casing, our engineers were able to develop a magazine that presents a round to the weapon with even greater stability. Increased magazine reliability results in overall improved weapon system performance.”
Note, however, that this only reduces magazine related stoppages, and does not address the issues with the M4/M16’s sensitivity to grime, etc. and issues related to its gas tube operation.