Mixed liveries were common on many types of "heritage" DMUS. From the many pictures I've looked at, mixed liveries seemed to be most common during the transition from green to blue.
http://www.flickr.com/photos/blue-diesels/8079537166/
Derby_LW_Sudbury by robertcwp, on Flickr
W59528_Weymouth_13-8-67_m by robertcwp, on Flickr
E59696_1971 by robertcwp, on Flickr
Cravens_nrPreston_18-7-67 by robertcwp, on Flickr
124_Hull_9-68_m by robertcwp, on Flickr
And some crop up in the blue to blue/grey period:
101-mixed_Hudd-ManVic_Saddleworth_3-9-81 by robertcwp, on Flickr
115_Rainhill_1980 by robertcwp, on Flickr
115_GerrardsCross_Aug-83 by robertcwp, on Flickr
But I'm sure there are plenty of examples of NSE mixed with blue/grey and so on.