Painting aluminum garage door
I'm going to make this a little simpler than some of the comments you have gotten and say that it's really as easy as priming and painting with a good quality exterior house paint.

I would clean the surface well from dirt, oils and other contaminants and then prime with a exterior oil based primer such as this Zinsser Cover Stain primer (random selection I'm familiar with, but just an example).  It comes in quarts and gallons and will probably be cheaper than a specialty metal paint.

Once primed, you can use a quality exterior house paint tinted to any color.  This is a very common way to paint the doors to match house and trim colors, and should be a long lasting solution.