Since fire coats the things it can't destroy in black opaque soot, it doesn't really matter if the colors underneath survive... they won't be seen until after someone cleans the sign, after the fire is over.
I would suggest using the sign's shape to convey its meaning. That way, even when it is totally blackened and silhouetted against a wall of flames behind it, its meaning will still be clear.
As for material suggestions, I would consider aircrete ( a mixture of concrete with a foaming resin ) which is extremely fireproof. Also, there are a lot of additives that you can add to concrete (and by extension aircrete). Gypsum or Quartz powders will make the sign reflect flashlight beams. Dyes and colored stones can be added to increase the sign's unsooted visibility and some to those dyes absorb sunlight and glow in the dark. The sign can either be made relatively attractive or garishly noticeable with very little additional effort.