Assuming the garage door is previously painted metal, it is clean, dry, sound, with no bare metal areas and prepared as detailed in the technical data sheet you could consider Rust-Oleum CombiColor Original.

This can be mixed to many RAL, NCS or BS colours in a satin or gloss finish.

If there are any bare steel areas these should be primed with Rust-Oleum 3369/3380 CombiPrimer Anti-Corrosion.

If there are any bare aluminium or galvanised steel areas these should be primed with Rust-Oleum 3302 CombiPrimer Adhesion Primer.