I guess that would depend on......

what kind of diesel you want to weather?  Lets assume it's a yard switcher as that is what I am familiar with.  First I would spray a flat dull coat on the side frames to give them some surface and get rid of the shine.  I would then spray a light coat of a mixture of dark earth color and maybe some grimy black.  The more weathered you want it to look, keep spraying light coats until it's the color you want.  I would then drybrush some dark rust followed by a lighter rust on the details to bring them out.  This is just my version so I hope some others chime in!

Thanks Nelson for the reply.

Thanks Nelson for the reply. SD24 and GP9 we're talking about.  The technique you described is something that I was thinking , too. You choose color of weathering considering an area you are modeling or function that loco has.

Cheers,

Jure, Slovenia

" I have some photos of prototype"

that's the first big step. Look at them and try to figure out what color shows up on the silver. sometimes it's rust color or sometimes it's dirt or soot color and usually it's a combination. Once you have an idea start with some light washes of acrylic paint and see how it goes. If you have an airbrush you could use light sprays instead of the washes.  You might also want to try some weathering powders as they are easy to use and not as permanent if you don't like the results. Check out youtube for weathering videos, they don't have to be trains, guys weather cars, military equipment,etc. too and all the methods are useful....DaveB