It's Blue. Green has not been a good coolant for Mercedes engines since the 80s. Coolant spec is 325.0 for this configuration (M273 engine in W221 chassis). From the Bevo site Bevo 325.0 Coolant Spec. Many choices on there, Zerex G48 may be the easiest one to find locally, but not sure where you are located.
As for the amount, it depends on your car. Please take a moment to fill out your info in your profile (cars owned, general location), it will help provide more specific answers.
This is from the Operator's manual of my 2008 S550, could be the same for yours or very similar, 2008 Operators Manual attached for convenience.
Placed an order for a bunch of stuff through a new supplier. Parts arrived yesterday and the familiar MBZ coolant jug looked somehow different. The fluid level stripe was dark so I opened one jug to take a look. Blue? WTF? Bottle is marked BQ 1 03 0004 Invoice says Q 1 03 0002 and "Can use...
I'll have to check on the screen name change, not sure.
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