Honestly that's a super cool idea but I gotta ask, did the color turn out okay? Tbh I'd be worried about it turning into a muddy brown or a weird gray. I have a bunch of those little sample pots in my garage and they're all different finishes, like eggshell and semi-gloss. Ngl mixing those together sounds like it could get lumpy or not dry right. Did your neighbor say anything about the final texture or how it held up in the sun?
My garage door is a custom mix from five leftover sample pots! It came out a really nice slate blue, not muddy at all. @craig.reese, I stuck with all satin finishes to avoid texture issues, and it's held up great for two Arizona summers now. Just make sure you stir it really, really well!