One small caveat to this whole thing. Be very careful how you blend the pastel chalk into the panels. Scratching can occur, and I recommend doing all your fading before sealing with Future. Blending can scratch the Future clear coat. Remember, before you seal.
As for color. Well, generally you want to go with a lighter shade of the same or complimentary color. In the case of an OD finish Yellow is close enough to provide a nice faded finish. I have experimented with OD, Green, Metallic, and various Blue finishes.
OD and Green finishes
Generally, I use Yellow for OD and Green finishes. I've tried lighter greens, but the colors that I have for the chalks are pretty generic and resemble a box of crayons so yellow, given what I've got, is my best bet. I'm sure plenty of art stores have pastel chalks in complimentary shades of OD and Green that would suffice.
Metallic
This was an adventure. All I really succeeded in doing here was making the panel look dirty. Which was not what I intended to do, come tothink of it I cannot remember what my intentions were, but I actually like the end effect. Ya gotta be extra careful here though in trying not to make a mess on other panels.
Blues
I use a light baby blue to fade the flaps on most of my sea based planes. Mostly over a dark sea blue finish. I've tried white on Blue-Grey and Intermediate Blue surfaces, but it either looked too,well, chalky, or you couldn't tell once sealed.
Remember, the most important thing is if you plan to seal your model with future or something similar like a dull coat. It will darken, so go with a brighter color than you thing you need, and careful on blending in the chalks as to not make scratches.