Database
[{"name":"Adam","country":"Argentina","colour":"Red"},{"name":"Betty","country":"Belgium","colour":"Blue"},{"name":"Clive","country":"Argentina","colour":"Red"},{"name":"Daniel","country":"Argentina","colour":"Blue"},{"name":"Edna","country":"Belgium","colour":"Blue"}]
Query
db.collection.aggregate([{"$group":{_id:{"country":"$country","colour":"$colour"},"names":{"$push":"$name"}}},{$replaceRoot:{newRoot:{$mergeObjects:["$_id",{names:"$names"}]}}}])