When you open a new gallon of paint that you won't use all of right
away, mix really well and pour off a couple of quarts into new quart
metal cans. You can usually find qt size cans cheap at paint stores
or on the internet.
I usually paint a car in sections and this has really helped keep the
color consistent, especially with metallics.
I mix the gallon well and pour off 2 or 3qts filling completely and start
using the left in the gallon can first to paint with.
It's a help with primers that settle quickly too.
It's almost impossible to keep pouring off the top with metallics
and expect the last qt to be the same as the first.
I usually have a qt left over on all-over paint jobs
so that last qt. is saved for future repairs .

Good tip! I do the same and also try to have a left over quart for future repairs. Your braver than me to panel paint metalic colors though!

Let me add to this great tip, INNERMIX THE CANS! Don't just pour out of the gallon and think all is good. Get another gallon can as well and pour half the gallon into the it. Stir up BOTH real good scraping the bottom of the cans with your mixing stick, then pull the stick out of the paint and examine the bottom of the stick to be sure there is no heavy toner sitting on the bottom of the can that you havn't mixed up. Pour a quarter of the gallon back in the first can and stir them both again, then pour a half of the first can back into the second can.........a couple of times back and forth and you KNOW you have the same color in each can.

Brian

Good advice Brian, That's another reason I try and pour it off
as soon as possible after buying it.
The paint store gets it shook up real good after mixing.
I always add a little to each can at a time, taking
several passes to fill them completely.