Blue may be the colour for United's successful Champions League campaign - but it has left City fans seeing red!
Tuesday's 4-0 hammering of Olympiakos saw United don their new blue away kit at Old Trafford for the first time.
But City's supporters reckon they should be the only Manchester club who wear blue.
Loyal fan Graham Astley of Wythenshawe steamed: "It's sacrilege. City would never lower themselves to wear red at home!"
Similar sentiments were expressed by Don Price of the City Supporters' Club Prestwich branch.
He said: "It's ridiculous. Their kits have gone through every colour of the rainbow. I'd be totally gutted if City ever wore red."
Blues fan Aide Dickson speculates the reason for United's blue look.
He said: "I think they've got an inferiority complex and are trying to copy the leading football team in Manchester. City would never dare to wear a red kit."
Terry Donaghue of Blackely asks: "Have United done it to blend in with the majority of Manchester's football fans?"
Ironically for City fans, two of the most memorable matches in their club's history came whilst their side were wearing the red and black of Manchester United.
That was the colour of City's away kit when they collected the Cup Winners' Cup in 1970 and when they beat Leicester in the 1969 FA Cup Final.