Wiktionary
a. Of a bright red colour, like that of a British mail box. n. A bright red colour, like that of a British mail box.
Usage examples of "pillar-box red".
The one to the left, green-painted, bore the legend 'Propellant' and below that the words 'On-Off: the one to the right was a bright pillar-box red in colour, with the words 'Safe' and 'Armed' stencilled in white on the left and right side of the box respectively.
The figure in the pillar-box red anorak arched as if kicked in the spine, threw up its arms, pitched forward, rolled once, and came to rest, half-obscured beneath the inspection catwalk.
The old 2CV, freshly pillar-box red after its latest re-spray, puttered through the near-empty streets of the bright and silent city, and rocked and rolled its way back down towards Gallanach.
Why don't you paint your lips pillar-box red and varnish your nails to match?