Every case is different. Last fall my, just purchased, Raleigh Pro Mk IV had a lot of surface rust on the top tube and down tube and the original frame clips for the rear brake were gone, replaced by DuraAce clips. That just said the bike had been ridden alot and 'put to bed wet'. It could have also been on a trainer a lot to as it appeared to be saltwater damage from sweat. I treated the corroded sections with rust converter and tried to touch up with nail polish. Close match but it really just looked blotchy. I wiped all that off and decided to spray just those two tubes.
I found a GM Maui Blue metallic Duplicolor that looked pretty close to the frame color. BTW this frame was not Blue Mink and there were no decals anywhere nor any sign of there ever being decals. It may have been a Worksop respray but the original Blue Mink was under the slightly darker top coat. So waddaell, it's not original anyway.
I knew that a slight color mis-match would be easier on the eyes if there was a joint of some kind between the two paints. So I masked off the lugs on each end of the TT and DT and masked the rest of the frame (the bike was totally apart for the renovation anyway). I light sanded those two tubes, cleaned them and sprayed according to instructions then cleared according to instructions. I was surprised at how close the match was when I pulled the masking. Better than a 10 foot job, maybe a 3 foot job.
Note: that paint was pretty soft at first even after 'baking' it in the back of the car on a hot day. Now, 10 months later it seems pretty tough and may last nicely. I've been living with it and have about 1000 miles on that bike now but I was pretty careful with it early on. I'm happy I sprayed mine as the damage was way more than a few scratches.
I'm enjoying the Mk IV experience now and will never represent this bike as 'original'. The paint suits the condition of the components and the intended use of the bike. I get this all 531 DB & rapid taper tubing thing. Great ride, great look. I may even get a couple of decals some time (maybe those cool '72 Carlton bits).