That is the color of transmission fluid.
The question is where? There is a transmission cooler that is integrated in the lower part of the radiator. Check the metal hose fitting that exits out of the bottom of the radiator near the passenger side for leaks.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car