Compare the product line and sheen before buying
Do not stop at the large color-name label. Compare the manufacturer, exact product line, and labeled sheen on the old and replacement cans. Sherwin-Williams says on its ColorCast Ecotoner page that its color formulas are calibrated for each of its products and sheens; that describes one brand's system, not every paint maker's. The practical question for the paint counter is: "Is this the original product and finish, or a substitute requiring its own formula?"
A sheen mismatch can make a touch-up appear lighter even when the base color is the same, according to the ColorSnap Match FAQ. Treat "same color" and "same finish" as separate checks. Photograph both cans and their tint labels so a paint specialist can compare them.
A new batch, or an old leftover, adds uncertainty
A later purchase is not guaranteed to match exactly. Sherwin-Williams' contact and FAQ page explains that paint products, tint products, and tinting equipment all operate within specified tolerances, so cans bought at different times may not match exactly. Ask the supplier to review the original label and purchase record when available.
Original leftovers are useful but not infallible: the manufacturer's touch-up matching article notes that stored paint can lose moisture, making a match more difficult. Before relying on an old can, ask whether its condition is suitable for reuse.
The existing wall may no longer match its original paint
Sunlight can change the finish you are trying to match. Sherwin-Williams' fading guide says colors can fade on sunny exposures and that interior coatings may fade near windows with significant sunlight. Matching the original specification therefore may not reproduce the wall's present appearance.
Surface behavior matters too. The touch-up matching article explains that moisture wicking into the surface affects how the touch-up dries. Ask whether the touch-up will use the same type of applicator as the original coat; the can alone does not control the result. If the problem is a repaired patch rather than a color question, see why a drywall patch can show through after painting.
Agree on a small trial and a fallback
A useful next step is one small trial before expanding the work. Follow the exact product's label and technical guidance, and ask the paint specialist when to judge the dried result. Do not improvise a thinning recipe: the manufacturer's own touch-up pages give differing dilution advice, so confirm it for the specific product.
Decide in advance what happens if the trial remains visible. Sherwin-Williams' contact and FAQ page acknowledges that repaired areas may stay noticeable and that painting the entire wall may be the best option. Compare a spot touch-up with repainting to a natural break; neither should be described as a guaranteed invisible match. More painting questions are collected on the painting topic page.