I get this question a lot, and the truth is I rarely use a color straight from the bottle/tube. I do try to as often as possible, as the color is easier to reapply later if I need to, but most times I have to mix colors. Unless it’s white, black, “rosy beige,” “opaque” red or “opaque” yellow, then the color is probably mixed.
I recommend using the little paint pots sold at craft stores. I mix up paint, close the lid, and it’s good for a few weeks. Heck, put it in the fridge and it will last even longer.