It wasn't to hide them while they were drinking, it was to prevent discarded cans from being seen from the air.

For many years, the majority of food and beverages for troops were packed in metal cans since they were convenient and durable containers. However, the metal surface of cans also reflects sunlight, and it was quickly learned that enemy reconnaissance aircraft could see those reflections. The enemy could gauge troop size by the number of discarded cans, and their movements by following a trail of them.

The obvious solution was to bury them, but that wasn't always possible in rocky areas or in the winter when the ground was hard. So, the canning industry developed coatings that dulled the metal or camouflaged it so they wouldn't reflect sunlight. Olive drab was the most common color used, but black and brown was used too.

And yes, beer was made available to soldiers. It was typically offered in mess facilities behind the front lines.