mixing paints

Hi I've bought Valspar Trade Walls and Ceilings Tough Matt paint, tinted with Blueberry Ice from B&Q -...

Hi All, Money saving/tight-arse question here: I'm just about to start painting my living room and have bought just enough (two tins) Dulux matt. I was planning on being very careful to avoid having to buy a third tin to only use a tiny amount. Good news... my mate has some leftover Dulux...