Given Awdry was quite keen on accuracy to the real world engines (see the falling out with one of the first artists) I’m actually surprised there isn’t more consistency with the liveries. The Culdee Fell Railway has all engines with the same purple livery, and I think from memory the steam trains on the Skarloey Railway are red, which is more accurate to real life as generally each railway would have its own livery.
I think earlier on when there were fewer engines it was less of a problem as they all had fairly distinct shapes being based on different real engine classes Garden and pre rebuild Henry probably being the least distinct.
So Reddit gold?