Old paint showing through effect?

[Valkyrion]

This might not be feasible for my project but I was wondering how to go about having previous colours showing through repainted army.

The main colour of my force will be very dark grey but as per their fluff they 'recycle' other space marine equipment. Being far too busy to painstakingly recolour this equipment I thought it fair to get menials to quickly daub the new stuff in grey leaving base colours showing through.

 

For instance, one might have a helmet stolen from a blood angel. 

Would you paint the helmet grey then line the recesses with red?

Paint it red and drybrush it grey?

Paint it grey and use a relevant wash or glaze over the top to hint at the underlying colour?

 

Any suggestions would be peachy, ta.

[NovemberIX]

Maybe daube some color paint at the wear edges of what's supposed to be underneath. Do little areas where it looks like paint flaked off in flat areas. Or you could do what I've done on some canvas paintings, paint what it supposed to be underneath, then go over it (hastily I might add) and paint the models gray.

[UltraRich]

I used a trick once with a Star wars droid kit. 

 

I sprayed the model with silver car paint which is cellulose and much tougher than the water based GW paints. 

I then over sprayed it with citadel paints using an air brush. I then took fine sand paper to the edges and it rubbed off the top colour leaving a very natural metal finish underneath. 

 

I appreciate this may not work for minis (especially if a helmets from here, a pad from there) but it does work well

[Kaedes Nex]

Weather the helmet using the sponge paint chipping effect, but instead of using black use the underlying paint color.

[Bung]

There are some other ways, if you own a airbrush you could easily work with the hairspray or salt method for chipping effects.

Or you could just spray some Tamiya Clear Grey on top of the basecolour.