A couple of hints here.
First, there is a transmission cooler in the radiator that, if it leaks a little, can turn the coolant colors.
Also, the SMALLEST little bit of oil in the coolant will appear to be alot.
Allow them to clean and flush the system then check on it. We have had to flush them 5-6 times with degreaser to get the oil out of the system.
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MERCEDES SPECIALIST 11 YRS