The Agent has produced coloured chalk of a hue called Australian Stone that matches a lot of sandstone and granite in Australia.
This coloured chalk is the best for the Grampians sandstone. Although you can still see it on the rock if you know what you're looking for, it is much less obvious than white chalk, and could conceivably pass as a natural variation in the rock.
The chalk is a loose power, consisting of 99.9% pure MgCO3, with 0.1% being the colour pigment of dyed MgO. Available in 100g resealable bags.