I don't know, Dive. They had a standard grey - very common among most railway companies probably because it was cheaper mass produced stuff for what were workhorses; no need for varnished ginish and attractive lining on a vehicle designed to carry stones, hay, parcels, cattle or cement.... The early H vans were this "normal" grey too (and at least one fitted one was standard green!). Colour photos (and memories) after about 1961 or thereabouts show H vans, palvans, and I'm nearly sure cattle vans, plus of course the then-new goods brake vans, all appearing in a light shade, still to be seen at the end of loose coupled traffic on quite a few wagons which had yet to be repainted brown.
The cement bubbles simply had this standard grey all over - in reality (and I remember it extremely well as I saw it every weekday) you might use LMS wagon grey. The chassis was grey too, and remained so when they were repainted orange. Once they became cream, the chassis became black.
The tops of those do look lighter in the photos, but they're taken on a dull day. There was no variation in the grey from couplings to chassis to tank top.
The roundel had a tan surround in these photos, with white lettering. It was the same transfer as used on H vans and palvans. Interestingly, any cattle trucks painted at this time, plus various other types of wagons, had an all-White roundel. I must check this, but from vague memory, newer guards vans had all-White.
Talking of guards vans, we see many variations (including in the UFTM) of the wasp stripes on CIE guards vans. There is one version that CIE used, thus if one wishes to have accuracy, all else is incorrect! That is, yellow and black stripes (not white and black); and the stripes were only on the protruding part. The section of bodyside above and below the protruding part was plain black. No older wooden ones ever had these stripes. Like earlier CIE guards vans, they were just plain grey.
By the start of the "brown" period, all repainted vans had wasp stripes, with none having a plain brown ducket.