I noticed a rust hole in the os wheel arch, so after filling and smoothing I applied grey primer. The car is sliver. I waited a week before applying the top coat ( Halfords alti silver specially mixed )
I simply could not get the paint to match the original, and was forced to remove the new paint as it was totally unlike the original. After several attempts I discovered by accident that I could get a very good match by spraying the touch up area with the spray can about 30cm away and at an angle of 45 degrees to the vertical. I let this dry for a few days and then applied the clear laquer. Result; quite a decent match nearly invisible in good light.

Congrats on the repair. The best thing is people keepining the older cars in good nick and on the road longer. Try waxoyling inside the bottom of the door skins, sills and underneath the car. :)

Worth removing sills if you intend to keep vehicle, + did you dig all the mastic out in the inside (boot etc) ?
see https://picasaweb.google.com/sailing4us/Rover600Rust?authuser=0&feat=directlink
I actually need to final finish mine (metallic cayman blue) sometime.

Thanks for that link. Fortunately in my case the rust was caught before it had advanced too far, cosmetic damage really.