As Sixto suggests, it could well be left over oil in the coolant reservoir from the earlier head gasket failure. These reservoirs are very difficult to clean out due to all the internal baffles. After replacing the head gasket on our 300TE a few years ago, I spent considerable time removing oil from the coolant reservoir (and the rest of the cooling system). When I purchased my 190E early last year, it was obvious the head gasket had been replaced, and the reservoir not cleaned properly. On removing the reservoir I found that some compartments in it were still full of thick oil. Since the reservoir was also leaking slightly at the filler neck, I decided to replace it (The 300TE actually ended up getting the new reservoir with the old one from it going into the 190E since they were the same part number and the TE is our car for long trips). Fortunately, all the oil was from the previous head gasket failure as I had suspected. Replacement is a viable option for a badly oil contaminated old coolant reservoir (they are not prohibitively expensive).

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.