[{variants:[{"color":"RED","color_image":"/red.png"},{"color":"red","color_image":"/red.png"},{"color":"BLUE","color_image":"/blue.png"}]}]

db.collection.aggregate([{$facet:{"colors":[{$unwind:"$variants"},{$addFields:{"variants.color_lower":{$toLower:"$variants.color"}}},{"$group":{_id:"$variants.color_lower",count:{$sum:1},image:{$first:"$variants.color_image"},}}]}}])