Assuming the external pipe work is steel, it was previously painted with a single pack paint, it is not exposed to elevated temperatures,  it is clean, dry, sound 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.

Any bare steel areas should be primed with Rust-Oleum 3369/3380 CombiPrimer Anti-Corrosion.