The coolant tanks used in MBs and most european cars are different from coolant recovery tanks used on domestics. The coolant tanks in domestics were not an actual part of the cooling system and were not under pressure. The reason for tanks in most MBs is to reduce the height of the radiator by placing the highest part of the cooling system not at the front of the engine compartment.
Most tanks are translucent, but often life's experience changes them to dark non transparent containers. In such case, if the coolant level light can't be trusted, one must open the cap and look inside to see whats there. AND unless one really knows what one is doing, it is best done cold.
Bosch Master, ASE Master, L1
33 years MB technician