I have a gallon of white paint left over from painting my garage

Take it back where you bought it from and ask to change it to green. Its free

Hang on to it as you might need for touch ups. If unused, and not a custom mix, you might be able to return it.

I agree with both the above - you can return it or keep it for touch ups. Is there anywhere else you can put it to use?

We save our unused paint in a safe place (think fire hazard) and where it won't freeze in winter to touch spots if something should happen we need to touch up what we have painted. Life happens, dings happen, damage happens. Fresh paint on door jams, window sills, or on the front door as well as along the outside and inside walls can make a huge difference in not needing a total repaint job. We maintain what we have with left over paint. Just a thought.

White paint will always be a good thing to have on hand for touch ups. IF it's oil based, you can press a piece of plastic wrap or waxed paper to the paint to keep it from forming a skin and drying out.

Save it in a place that won't freeze. You never know when you'll need it as touch up.