[{"Lookups":[{"REG":"ABC","RBG":"Green"},{"REG":"DEF","RBG":"Red"}]},{"Lookups":[{"REG":"HIJ"},{"REG":"KLM"}]}]

db.collection.aggregate([{$project:{matchingLookups:{$filter:{input:"$Lookups",as:"lookup",cond:{$eq:["$$lookup.REG","ABC"]}}}}}])