The paint you get in a tin varies a little depending on the batch - even if you get exactly the same colour from exactly the same brand - so unfortunately you’ll never be able to get a precise match (pro-tip: if you suspect that you’ll run out of paint whilst doing something, go buy another tin and mix it with the one you already have - it’ll be much harder to tell you ran out).
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The paint you get in a tin varies a little depending on the batch - even if you get exactly the same colour from exactly the same brand - so unfortunately you’ll never be able to get a precise match (pro-tip: if you suspect that you’ll run out of paint whilst doing something, go buy another tin and mix it with the one you already have - it’ll be much harder to tell you ran out).