???

Actually Super I wouldn't be at all disappointed here, in fact I would consider it an opportunity. My reasoning is that in 'real life' (god forbid) a goods train will rarely have cars that are identical in terms of colour, some will be bright and shiny, some dull and faded, some rusty, some maybe even with grafitti these days.

So you could either run them as they are mixed in order so they look random or and here's the opportunity for you, get some 'weathering' powder or paint and give them a little coat of that. This will tone down the differences in colour and give them that worked grimy feel. If you really wanted to get realistic you can even get stuff that will give them the look of having some rust on them. Applied judicisiously say around the edges/seams of the wagons they could be made to look pretty realistic. If you want something like that of course