The battery on my Honda was green/black/red depending on the state. When healthy or fully charged, it was green. When partially discharged, black. When discharged, red. Not sure about the Mercedes battery, but they're usually a function of the specific gravity of the acid in the cells and the balls "float" at different gravities based on charge state. If your battery is similar to the type stated above, it maybe getting towards the end of its life. A black window in a battery of that variety can still give good service for a period of time, but it's capacity will be reduced.

The battery shown in the picture appears to have caps, but whether or not they come off is unknown. Serviceable type batteries tend to have a cap for each cell (6) or a cap for each group of 3 cells that is raised with a pry slot for a screwdriver.

It's worth noting that when a battery is low on water it's on borrowed time. The most common cause being overcharging.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)