a lovely loco-
i know what you mean tho about bright orange and yellow locos
i suggest some artist chalks for a starter-
i have two sets -one in grey tones form black to white
another in browns and oranges
i scrape some chalk off using a knife either onto the model or into a cup-then brush on using a soft brush
completely reversible unless you fix it with.......artist fixative-typically for chalks and pastels-fixative WILL however greatly diminsh the apparent coloring-so you can either do it heavily-or do more than one 'coat' of chalk
the nice part about yellow and orange is the contrast with the chalk will be dramatic
if you like too-theres the old spray gun-mist weathered black or mud
i like mud for the lowest portion with a bit more the darker stuff on the leading faces of the loco with less on the sides -touch up details with a wash of black or rust-depending on what the contrasting color might be underneath
i feel less is more-it takes very little to tone things down on orange and yellow-
-and it will be a subtle factor not detracting from the beauty of the model