Red

You may be looking for the Seventh Doctor audio story.

Red was a colour used by humans, in the form of red alerts, to indicate danger. The Ninth Doctor said non-humans regarded it as "camp". (TV: The Empty Child)

Racnoss, (TV: The Runaway Bride) Blowfish, (TV: Kiss Kiss, Bang Bang) Zocci (TV: Voyage of the Damned) and Posicarians had red skin. (TV: Mindwarp)

Red Daleks and New Dalek Paradigm's Drone Daleks were identified by red casings. (COMIC: Eve of War, TV: Victory of the Daleks)

In 1893, Madame Vastra and her partners, Jenny and Strax, investigated "the Crimson Horror" — a mysterious condition leaving victims with red skin and preserved like statues. (TV: The Crimson Horror)