Why not drop a small metal nut or bolt or screw in to the paint tin, replace the lid, then give the tin a dam good shake about, till the rattle noise gets really clear, then remove the lid and use the paint.
its saves trying to stir a full tin, which can cause spillage if you are not careful, its not messy, easy to do , plus you simply shake the tin before each use to ensure full mixing. then once empty, you transfer the "shake weight" you used in to a fresh tin of same colour to prevent colour bleed .
Paint mixers are good, dont get me wrong, but just look at all the cleaning of kit you have to do afterwards, that uses more rags, thinners and time than really necessary.