The dieseline would have browned off the yellow much as WD40 will brown colours. Yellow never seemed to be a particularly durable colour on the BG 800s and 830s.
Why didn't PB steam clean the locos and maybe PK can tell us what they did at Pt. Lincoln?
There were a lot of things different at Peterborough. For example, D servicing was done over two days rather than one as it was at Mile End except I can remember a loco being late in and doing a C service in an afternoon including bedding and testing of generator brushes which was about a 25 minute job in itself... that was heavy going! Maybe it was in deference to the nature of the railway that a loco could be in as late as an afternoon and still expected out. Mile End always had the locos ready of a morning and cool if not cold for 5 days a week of heavier servicing.
I cannot answer with any degree of authority why the dieseline was used... maybe steam was ineffective on the local grits or perhaps it was a carry over from steam days when dieseline was less than 10c a gallon and water was needed for locos?
Open floor for now...