Hi there Zephira I just happened to notice this old thread by accident today in July 2019 so apologies for this late response. I only joined around 2010.
The Blue train sported mainly LX sleeping cars and one diner in its consist on part of its run (probably uncoupled in Laroche Migennes before the big hill on the PLM which was between that town and Dijon). In its early times it probably sported some S type sleepers which were most likely replaced by Lx as they became available. The old original LX livery would be good for very early versions of the train (powered then by PLM pacifics in their original condition therefore a good match to the Aster PLM pacific). This livery was blue with ornate gold leaf lining as per original brown Flèche d'Or pullmans, however this was done in gold leaf contrarily to the J&M models where it is painted in yellow, of course, hence the name of the train which wasn't official for many years.. A carry over from the days of wooden passenger cars, all trains in the pre war era (PLM era) had to have one baggage between the engine and the train and one at the end of the train. The blue train often used wooden baggage cars of both the bogie versions and the six wheel ones (of which I have a model) it later received the all steel baggage like the J&M one. After the war it was hauled by SNCF 2-D-2 9100 electrics (very nice Fulgurex model available if you can find it. I scratch built my own one which inspired Fulgurex who had their stand right next to the showcase it was in at a Paris exhibition...) to Dijon then in 1952 to Lyons as the electrification progressed south. Further out to Marseilles it was hauled by the brand new 241P as offered by the Twerenbolds and Aster a few years ago. From Marseilles on to Vintigmillia the very mountainous line along the riviera it was hauled by the liberation mikados 141 R (Aster model oil fired version fine); as these made up on the hill climbing what they lost on the level sections. At this time it often ran with one baggage only as the old restriction of two baggage cars had been lifted by then. However it did feature a very unique bar-dancing car which was obtained from the body of a Côte d'Azur pullman (7 windows type without kitchen) painted all blue to match the rest of the consist (this model was done by J&M). After 1932 the cars were blue with yellow simplified lining as per J&M models. The fame of this train is what made the CIWLT change from the British inspired dark umber below the window sash and cream to blue and cream or blue for sleepers, diners and bagage cars. Now of course the Blue train had some through cars north of Paris hauled on the NORD railway with Super pacifics on the point (Extremly rare Bockholt model in electric) then the Chapelon pacifics. It was sometimes combined with two couplage (one pullman with kitchen+ one sans Kitchen) of Pullmans on the Golden Arrow. So before 1934 with a Colin or De Caso super pacific (The chapelons were built for the Nord in 1934 and weren't really operational until 1935) you could do a train with the Flèche d'Or container baggage, two couplages of Golden Arrow Pullmans in brown and cream and a few blue LX with the original striping + one baggage bringin up the rear, which would make quite a beautiful train. If only Fulgurex had turned out that Super pacific it planned to do back in the late eighties...
By the way this brings up an information I often repeat to American modelers: The Brown Golden Arrow pullmans were repainted in 1932 to the blue livery with simplified lining, the Chapelons really began to appear on the NORD in force in 1935 so don't match them as it isn't historically accurate, at least for those who like me are historically inclined in their modeling. Of course you do what you want on your pike.
To answer you on the livery of these cars it is likely that they did sport white roofs for the early livery version and stripping, however it was later changed to battleship grey roofs these both stayed that way for less than six months then gradually became black which is what we remember in SNCF days. At one time I sprayed some black lightly on the grey roofs to show an intermediate sittuation. But I since have stopped that practice at it makes touch ups quite impossible to do in case the paint chips...The best source book on Wagon lits is the one by Gerard Coudert edited by La Vie du Rail called La Compagnie des Wagons Lits. He got most of his info from Comault who was the best specialist on CIWLT in the old days... I hope I didn't bother too many with my "mister grumpy" historical facts, Cheers!