If it's not too big of a spot, I'd use the old paint and, if it isn't satisfactory, you can always paint over it with the new paint. I've been through this recently. The cans of paint in the garage all pre-date my purchase so pre-2003.
The outside paint color was in a can with the lid rusted shut. I took it to the local paint store (a real paint store not Home Depot etc). They shook it up, punched a hole in the can and poured it into a clean can; all for $2. All of the paint solids had sunk to the bottom and they got most but not all of it so the paint was a little lighter than the paint. But the color had also faded over time and it's a pretty good match.
The trim color and the front door also had to be done. I had the paint number, but the door had faded and the color number was for a discontinued brand/type. They used it as a starting point, but had to do trial and error to get it to match. I took a bit of the door that was flaking/delaminating and they were able to match it pretty closely. They did an amazing job for me and, while the paint may cost more, they have a customer for life based on the service.
On the other hand, I have a second bedroom that was a child's room before I moved in; it's purple. There were several white spots that I tried to touch up. My mother is an artist so I had her bring her paints over and tried to match the color. My patience ran out way before we had the right color. It looks terrible so I have to take another shot at it (or repaint the room and that ain't happening...). Moral of the story, I am reminded why I hate to paint.
Sue
