Peter,
I think you will find that Sir Nigel Gresley is only one out of a class of loco, namely the 'Pacific' class.
Also locos were repainted many different colours during their working lives.
The Mallard was one such of this class, and that broke the world record speed for a steam loco.
They also come undressed as in the picture below, without the streamlining. Same class of loco, different clothes.
John