[{"_id":"1","someArray":[{"_id":"1.1","color":"RED"},{"_id":"1.2","color":"GREEN"},{"_id":"1.3","color":"GREEN"}]},{"_id":"2","someArray":[{"_id":"2.1","color":"WHITE"},{"_id":"2.2","color":"BLUE"}]},/** many others here...*/]

db.collection.aggregate({$match:{"someArray":{"$exists":true,"$ne":[]}}},{"$unwind":"$someArray"},{"$set":{"someArray.colors":["$someArray.color"]}},{"$unset":"someArray.color"},{"$group":{"_id":"$_id","someArray":{"$push":"$someArray"}}},{"$merge":{"into":"collection",}})