IMHO during the Spanish Civil War the Communist Party in Spain was exercising real power (largely through and due to Russian influence and aid) but an essential part of that was that such power was exerted through non communist leaders such as Negrin, as such the 30's communists would have kept the Republican flag. Had they taken or retained power in the 40's (eg by a failure of Overlord and eventual Russian liberation of Europe) then the flag would have perhaps had a workers symbol imposed on it aka Roumainia.
Red has for centuries been a part of the Spanish flag so it wouldn't be too hard to incorporate trad Spanish images with communist ones.