db={"collectiona":[{"_id":1,"primary_color":"red"},{"_id":2,"primary_color":"blue"}],"collectionb":[{"_id":1,"available_colors":["red"]}]}

db.collectiona.aggregate([{"$lookup":{"from":"collectionb","localField":"primary_color","foreignField":"available_colors","as":"matches"}},{"$match":{"matches.0":{$exists:true}}}])