I want to paint my tiny bathroom, but I'd only need about 1/3 of a 2.5 litre tin. I have a partial tin of silk that's a bit lighter than what I would like. Can I mix in a somewhat darker mat sample pot...or 2...to get the colour I want.
Yes emulsions can be mixed perfectly well.
But..... adding a small sample pot is possibly not going to do much for the colour. It'll probably just make it a bit "muddy".
What you really need is a colour pigment/tint.
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Also........... with any colourants, you must mix the whole amount you need in one go. Otherwise, you'll never be able to "touch-up" any later damage.
If you have a Homebase near you, Pasta, they may still sell paint pigments in syringes. The one we had in Burnley used to sell these but it was a good few years back.
As The Builder says, be certain you have achieved the colour you desire so that some paint can be saved for those odd 'touch-ups'. Maybe add a little water to the two paints - both to facilitate a smooth mix and also to increase the volume a little.