Color Matching: Not an Exact Science!
It is hard to color match old paint especially if you don’t know the exact brand and color of the old paint.
The first thing you should do is peel or scrape off a piece of the wall and take it to a retailer like Sherwin-Williams. They usually do a good job at color matching.
If you succeed at getting a good color match, make sure you use a primer. Just have the primer tinted to the paint color for maximum coverage – apply one coat of the primer and one coat of the paint.
Not only will you now have the right color on the walls, you will also have a primer coat and a finish coat on the walls.
Good luck on your paint project!