I was recently sent a vintage postcard with this pre-stamped Red Barn postage.
Of course to be brought up to the 35 postcard rate, two definitive stamps were added. The 3c definitive is red... well maroon maybe.
And the new 10c definitive issued Jan 17 is red... in the form of pears.
One of my favorite red stamps (well, there is a tiny bit of blue and quite a bit of gold, but it's mostly red) is the Lunar New Year stamp from 2008, the first in the current series of twelve being illustrated by Kam Mak celebrating the clever rat.
On the topic of Lunar New Year, the Year of the Monkey stamp has red peonies on them, and the USPS stamp page covering the stamp has a lot of red.
Happy St Valentine's day.