chams:
you want to reset the texture to a single color bright texture
disable the z buffer so you can see through walls, set their texture to 1 color
enable z buffer so that the guys in front of the wall are selected
set them to another color.
yay chams.
you didnt search very hard btw, theres tons of tuts so YEA im not giving you the step by step - you said a short description:
disable z so you can see through walls = chams.