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I will add one more comment. Often you can do a single plane (one wall) stopping at a physical edge and not have any problems. We are accustumed to seeing slight differences on different walls as the light will not be the same on two different walls. ....
>OP, Even if working out of the original can, over time the paint on the wall will begin to oxidize and darken a bit, making an exact match impossible. Unless you know the paint brand, line within brand, and color you're unlikely to even get close. Add to that the fact that the manufacturers have had to reformulate products to conform to VOC regulations and thus even the same color is no longer exactly the same. The answer to your question is yes, you are expecting too much.
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