I believe during the 60's there were 2 independant operators (J.H. Hammel & Mowbrays) who shared the service hence the different livery (similar to Economic of Sunderland: Wilson & Anderson)
Here's the Green/Cream livery of Mowbray:
http://www.flickr.com/photos/johnmightyc...6370990820
and:
http://www.flickr.com/photos/johnmightyc...6370990820
Red/Grey Livery of J.H. Hammel:
http://www.flickr.com/photos/solenteer/8067192631/
Maybe someone can show these rather than the link
Gypsy Queen was also an operator bought by Northern in 1989 (Operated services around Langley Park & Durham):
http://www.flickr.com/photos/johnmightyc...6370990820
Diamond - Stanley/Durham