There were rules as such, burnthebox, but like today (think white logos on ICRs) there were the odd exception.
Paint shades were as uniform as today. Wear and tear tended to dull some of them down, of course, and different lighting in photography made things look very different indeed at times.
There was an old adage at Whitehead years ago, in regard to the GNR's loco blue, that "well, I've heard that they just went to the local blue paint shop and got whatever was there". Thankfully, this is abject nonsense, possibly propagated to justify the RPSI's earliest attempt at repainting 171 in a blue many shades too light. The same has been said of CIE green. Both Dundalk and Inchicore had actual paint laboratories (I knew the last surviving member of staff at Dundalk) who went to great lengths to ensure uniformity by looking at what additives the paint might have; they mixed colours themselves on site.
In terms of logos, there would be a general rule that in 1945-55 (older, darker) CIE green, two snails were on bogie coaches and one on 6-wheelers. Post-1955, with lighter green, one was used always.
Numerals were always on the left.....