There are many factors which are important when matching new paint to old. The size of the "chip" that you bring in to Lowe's is important. So is the texture of the sample. So is the type of new paint that you are purchasing (satin, semi- gloss, gloss, latex, oil, etc.)
Exactly matching paint can sometimes be very difficult. Still, Lowe's, Home Depot and others will generally guarantee the match, within reason. Take back a good sample of the old paint, a sample of a surface painted with the new paint, and the can of new paint. Be polite, go there when they aren't likely to be super busy, and ask if they can attempt a better match.
You may need to skim off a bit of paint from the new can to make room for the additional tint that they will add. It is also useful if you can have a sample material which has been painted with both the old and new paints so that the differences are obvious when viewed side-by-side.
Don't attempt to tint the paint yourself unless you have no other option. Even so, be advised that sometimes (uncured) paints are incompatible. It is possible that you may add a bit of flat white latex to some tinted latex paint and the paint is ruined. That doesn't happen every time, but it does happen. Two different latex (or oil) paints can have incompatible solvents, emulsifiers and/or binders.
If Lowe's won't help you, you may be lucky as I am. I have a very, very good relationship with my local True Value hardware store. Several times I have gone in (at a non-busy time) and the manager has retinted some paint I own. Often it is paint which I didn't purchase at his store. Why is he so considerate? Because I am a consistent & loyal customer, I am a friendly customer, and I send in a lot of referral business. I have worked out similar good & consistent customer relations with NAPA, Harbor Freight, Benjamin Moore Paints, the corner grocery, etc. It really helps when you walk into a store and the manager immediately smiles and greets you by name.
I like the approach of one of our local Benjamin Moore retailers. After they custom match new paint, they put a dab of the new paint on my sample of the old paint. Nobody in our family can identify the spot which contains the new paint! That is one method of letting the customer leave the store with a rather high confidence that the paint will match. If it doesn't, they'll work their butts off to make it match.
Good luck, Gideon