Kind of hard to describe standard colours when the mushrooms don't comply.
If all the specimens of a type were exactly the same colour then you might be able to describe that colour exactly but they don't.
That is why you sometimes see the colour given as a range of colours.

Also note that with most mushrooms the colours change depending on age and on how dry or wet the mushroom is.
For an example you can take the common Agaricus bitorquis.
The gills are usually quite pale when young, and as the mushroom gets older they turn darker until they are quite brown, almost black.