That second photo there has a glimpse of the short-lived NSE precursor "Jaffa Cake" livery - from the period between the creation of the "London and South East" sector and the launch of the NSE brand.
If I recall the London & South East sector planned three variations of the livery. "Jaffa cake" was the Express livery, as used on the 4-CEPs and 309s, as well as the earliest 421/7 units to be refurbished at Eastleigh. After the launch of NSE 4-CEPs kept being painted in "Jaffa Cake" for a short while, but the 4-CIGs quickly lost the livery.
The other two versions were to be for outer suburban and inner suburban services, one being blue based and the other green (I'm unsure which way round they were to be).
Were the 442s the first class to get the later variant without the grey/ a lighter grey on the lower part? Some of the 319s used that version, as did all the Networkers (which had a thin band of white between the blue and red at the ends)
Can't recall if it was the 321/3 or 442 units that first carried the later variant. It was also applied as you say the 319/1 units with First Class, the 159 units (South Western Turbo), 165/166 Turbo units and the Networker EMUs. Interestingly, the Class 456 units gained the earlier "more grey" livery like the 319/0 units, which suited their use on suburban services on the South Central alongside 455/8 units (that they worked in multiple with) and 319/0s.
If HSTs had been cascaded to the Waterloo-Exeter route (as was mooted at one point) it would have been interesting to see how NSE would have been applied to the power cars.