Matching the radiator to the wall colour is the most common way to make it recede, while a contrasting bold colour is a deliberate way to make it a feature instead.
Filtered broadly across neutral (blend-in) and vivid/jewel-tone (feature) colours, since both are legitimate real strategies.
A radiator is a mid-sized object fixed to a wall, and the question is whether it should disappear or stand out. Painted the same colour as the wall it recedes almost completely, because the eye reads its outline from differences in colour and lightness; a contrasting colour turns it into a feature. Heat does affect paint, so any colour needs a finish rated for radiators.
What the filter selected: 100 of the catalogue’s colours pass this test. Their relative luminance — roughly the share of light a surface in that colour would reflect — runs from 0% to 100%, with a median of 20%. 33 lean warm and 32 lean cool.
Red
red · medium
Crimson
red · medium
Scarlet
red · medium
Burgundy
red · deep
Maroon
red · deep
Brick Red
red · deep
Cherry Red
red · medium
Vermilion
red · medium
Fuchsia
pink · medium
Magenta
pink · medium
Orange
orange · medium
Burnt Orange
orange · medium
Undertone caution: two colours in the same family can still read very differently once on a large surface, depending on their undertone — check against your existing flooring, furniture and fittings, not the swatch alone.
Lighting caveat: a colour can look noticeably warmer or cooler depending on natural and artificial light through the day — see the lighting guide for what specifically changes and why.
Finish caveat: matt, eggshell and gloss finishes reflect light differently, which can make the same colour read lighter, darker or more saturated depending on the finish chosen.
Always view a physical sample in the actual room, at different times of day, before committing to a large surface.