Google "DuPont Imron color chart PDF", find whatever is closest. I just went to auto zone and found whatever was close. You can drop some bucks and have an auto body supply shop mix up the touch up paint for you.
When seeking paint for touch up, I asked two body shop paint supply houses for a custom match.
One shop wisely would not try it and claimed he needed a larger patch for his computerized "eye" to get a good match.
Another local shop that also custom mixes paint to match for body shops gave it a go but the results were not good. It seems that the bikes original paint had pearl added that was very difficult to match.
He only charged me for his materials ($20) and I was grateful that he tried.
I suppose it depends on the original color and the skill of the technician that selects and mixes the formula.
Often I carefully sand and rub the rusted area to bare metal until it takes on a bare metal silver sheen. Then either oil or seal that area.
Since I am careful to keep my bikes out the elements, this method works fine for small areas.
While protecting from rust, "touch ups" tend to jump out at your eye. But it all depends on the color, size and location.
Since I am a relative Newby at this I am curious what others here tend to find works well.
I use acrylic paints from Micheals Craft store. They have the little bottles of craft paints that sell for about $1 ea. They have a nice selection of metallics too. Buy several and mix and match. After touching up I go over with a good wax.