The coaches in that picture at Manulla need to be looked at in the context of one lot being in shadow, with t he ones on the right having bright sun shining on them. Like any colour, that will result in a totally different look in photographs. In reality the difference, which as aclass007 says looks considerable, was nothing like as different. The post 1990 shade was more orangey (i.e. less "tan") rather than darker or lighter.
While I know this isn't what is being referred to above, but as an aside to this, you will often hear it said that CIE's tan / orange / "golden brown"(!), or the GNR(I)'s loco blue, or earlier CIE green were, respectively "a million shades" of the various colours at different times. That's myth, usually borne of simple lack of accurate research. Actual post-1990 shade paint exists on some RPSI Cravens; the loco blue used at Whitehead is accurate, the light CIE green on C231 on the DCDR is accurate, and actual darker CIE green exists on Maedb in Cultra and on the flying snail crest in Enniskillen railway museum.
When referring to colour photos, it's important to consider lighting, the quality of the film (judge by surrounding scenery, sky colour, people's skin colour, plant colours), and the likely degree of actual weathering or fading of the subject matter in the photograph. Reference to several photos will usually give a better idea than one, and will highlight differences (or even cast doubt on theories that there were any!). Eyewitness accounts can be extremely reliable, or totally unreliable / plain wrong, depending on the artistic eye or memory of the beholder, or even the level of interest they had when they initially saw it.