Red

You may be looking for the Seventh Doctor audio story.

Red was a colour Rose Tyler 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, Blowfish, Zocci and Posicarians had red skin. (TV: The Runaway Bride, Kiss Kiss, Bang Bang, Voyage of the Damned, Mindwarp)