Some koalas may appear to have a reddish hue, but this is due to the sunlight reflecting off their fur.
Therefore, while there are variations in shade, there are no other colours for koalas besides shades of grey to brown.
What other colours are there for a koala than light to dark fur with brown stains on their chest?
Koalas only come in shades of grey to brown. While some may appear lighter or darker than others, there are no other colours. The "brown stains" you refer to are actually patches of bare skin on their chests called "bald patches" which are common in adult koalas.