It changes color, depending on how far the end of the Queste is.
Alther Mella recited a poem to Septimus about the colour of the Questing Stone -
Blue to get ready,
Green to go.
Yellow to guide you through the snow.
Orange to warn you that over you'll go.
Then Red will be the final glow.
Now seek the Black; There's no going back.
Tertius Fume usually Caused apprentices to draw the Questing Stone. There were 21 Stones and the last one was drawn by Septimus Heap in Queste. One was also drawn by Syrah Syara. It is revealed in Fyre that Septimus still has it.