I have weathered some of my freight rail blue locomotives using good old metho. It does make the paint go a lighter shade.
Just a bit on a cotton bud and wipe downwards refreshing the metho as required. However, go steady and do not get carried away by the process or you might take it too far.
My advice is to do a light job and let it sit overnight and then have a good look at it under your layout lighting, not under sunlight (unless your layout is outdoors).
Taking paint off to weather a loco is no different to adding paint for the same reason so you go about it a bit at a time.
I did an 80 class and at first I thought I went too far..... then I saw 8050 at Werris Creek and realised that I did not go far enough.
The best thing is though, if you can get hold of smashed shell, try it on that first if you are not sure.
Oh, I washed the body shell with soap and water between each of the metho treatments and I removed the windows just in case the metho fogged them.
It worked for me and I have no doubt at all that others will have their ideas as well.
Regards and respect