Relying on memory for weathering can sometimes distort things. Sometimes I see a photo of a steam loco in the fifites and think it looks clean-ish but can see the dirt clearly yet my Dad will comment that they'd have regarded that as being clean!
Similarly with early diesels, they did weather differently from diesels post steam. One thing though is normally that there's a sheen beneath the dirt (only when absolutely filthy did the sheen disappear, but that seems to be the extreme early on).
D275 passing Brough West SB (which would have been a lovely job - no level crossing for a start!) - the loco is dirty, the roof is filthy and the green is clearly discoloured in places but look and you can see a definate sheen beneath the dirt, coming through.
http://www.flickr.com/photos/blue-diesels/8135786107/
At Hull D201 in 1959 - looks quite presentable. Signs of use and note how the dirt above the cantrail grilles is left undisturbed by the washing plant!
http://www.flickr.com/photos/blue-diesels/3722380547/
And an appropirate location too. Road dirt has begun to creep up the bodysides but the loco is well turned out and you can even seen a proper shine at cantrail level.
http://www.flickr.com/photos/andersley_images/6065434168/in/faves-62338634@N06/
The loco in my signature is abosolutely filthy, but the weathering is applied over a gloss(yes, gloss) top coat. To really capture diesels and how the weather takes, I think, often more careful observation than for steam. As the diesel age progressed and locos went longer between repaints then you find that paint finishes naturally dulled which can lead to all sorts of different effects. My own 1992 period brings all sorts of contrasts between locos and their states, which really is all part of the appeal for me!