[{"_id":"605cc8d0c4f53fb6b67babb3","color":"green","date":"12/13/2020"},{"_id":"605cc8d0cb5012cd695596e2","color":"brown","date":"6/23/2014"},{"_id":"605cc8d02977e3b06881da30","color":"brown","date":"3/15/2020"},{"_id":"605cc8d0ae24fa0fcbf9116b","color":"brown","date":"4/17/2019"},{"_id":"605cc8d0a881e0cf6922de4e","color":"blue","date":"9/09/2019"},{"_id":"605cc8d0d510ee291f78ca8f","color":"blue","date":"12/16/2020"},{"_id":"605cc8d077af8a71550210c8","color":"brown","date":"6/20/2016"},{"_id":"605cc8d037c63e7b7255dca7","color":"blue","date":"12/06/2019"},{"_id":"605cc8d0a319b3a9c2cf1551","color":"green","date":"7/17/2016"},{"_id":"605cc8d0c7ea70260bb1ddc0","color":"blue","date":"3/09/2018"},{"_id":"605cc8d0c105e546d8c09b29","color":"green","date":"7/07/2018"},{"_id":"605cc8d0c513547bbcb9ad68","color":"green","date":"10/08/2014"},{"_id":"605cc8d004fb759228cd2008","color":"green","date":"5/14/2019"},{"_id":"605cc8d0d2ba723053e3f094","color":"green","date":"6/28/2020"},{"_id":"605cc8d079a9b904d8fe1ba6","color":"blue","date":"2/18/2015"},{"_id":"605cc8d03fa09505c66b24d1","color":"blue","date":"4/11/2016"},{"_id":"605cc8d0702e37d77ee067c1","color":"green","date":"11/10/2018"},{"_id":"605cc8d071a3dc314f9fd7ae","color":"green","date":"8/06/2020"},{"_id":"605cc8d0b915642d1f40a5c8","color":"green","date":"8/26/2020"},{"_id":"605cc8d09b1ef5d12c67dd80","color":"blue","date":"9/08/2020"},{"_id":"605cc8d07676cdf8cc684de7","color":"green","date":"8/14/2017"},{"_id":"605cc8d03b2b404027ed5a69","color":"green","date":"11/24/2014"},{"_id":"605cc8d037a1e94af532c5eb","color":"blue","date":"6/25/2020"},]

db.collection.aggregate({"$project":{color:1,month:{"$substr":["$date",0,{"$indexOfCP":["$date","/"]}]}}},{"$group":{"_id":{month:"$month",color:"$color"},"count":{"$sum":1}}},{"$project":{month:"$_id.month",details:[{color:"$_id.color",count:"$count"}]}},{$project:{_id:0,month:1,color:{$arrayToObject:{$map:{input:"$details",as:"pair",in:["$$pair.color","$$pair.count"]}}}}},{"$group":{"_id":"$month","color":{"$mergeObjects":"$color"}}},{$replaceWith:{$mergeObjects:[{_id:"$_id"},"$color"]}})