Crimson and white was indeed what was in the back of my mind. But early on the LLSR proper changed, as you suggest, to brown & salmon, as below.
No hard information on the very first LLSR carriage livery has survived, but the LSWR livery was indeed brown (lower; probably a dark chocolate shade) and a salmony pinkish colour above that with roofs originally white, but these would have weathered within a single trip to a lightish grey! Below are details, though the lettering and lining might have been different.
http://early-lbscr.co.uk/lswr1520/Livery specification for 1520.pdf
I can't help feeling that the ends were more likely to be brown, but I have no hard information on this.
I suspect that the all-black livery came into being about 1910. Lining on the black paint was in red, which probably looked quite smart, if somewhat funereal, when new. In post-war times, or maybe from the 1930s, everything was plain wagon grey.
(Irish railways have always seemed to have a love affair with grey! Today its the De Dietrichs and 071s....)