metallic needs to be painted all at once or it will not match. i had to fix a car i painted after it got scratched on the quarter and i painted it with paint from the same can as i originally painted the car with and it didnt match. i did it 3 times before i got it right and ended up getting out the original gun i used also. humidity changes it. also the amount of reducer you mix the paint with and the air pressure can make the metallic apear different.

another thing is , when i paint metallic i change direction that i apply the paint each coat. one coat may go a cross the panel from left to right and the next from top to bottom . so the 1-2 coats look like an X over each outher then the 3rd i usually either go back to left to right or diagional. i have never had any metallic streaks in any of the cars i painted that way. not sure if everyone does it this way or not.