Soft solder will turn dark gray on it's own with some time. They were most probably done with 50/50 or 60/40.
The new lead free solders will stay nice and white bright forever for you though. One reason I don't care for them on rib work.
If the problem areas are those thin lines of solder that appear as gap fillers between the rib and bbl where the fit of the parts was less than optimum,,the above will take care of the problem pretty well for you.
A touch of some cold blues will darken them too.
Brownells Oxpho blue will darken lead soft solder some. Enough to take the stark contrast of the line away.
A tooth pick with the end hammered lightly to create a brush or wick effect is handy to apply the tiny amount to the area. Plus the Oxpho blue doesn't 'after rust' like many cold blue will unless you flush them off w/water.
If the problem is patches of excess solder from the tinning operation originally done that are still visible on the tubes & ribs where they meet. I'd go in and remove them now.
You don't have to polish the entire set of bbls back down, just do those small areas and keep going with the rust blue. The color will match back up in a couple of coats.
Metal scrapers are good to clean out the tight area between the rib(s) and bbl.
Quicker and much more efficient than using a file or abrasive paper.
Using a V shape scraper with light hand pressure you can get the point right into the solder joint. Then rolling it to either side, clean the bbl or rib edge off w/no added work needed.
