The Hue command returns the hue of a Color object, which is the angle scaled from $0.$ to $1.0$ around the RGB color wheel where red is $0$, green is $\frac{1}{3}$, and blue is $\frac{2}{3}$.

$c≔\mathrm{ColorTools}:-\mathrm{Color}\left(''red''\right)$

$c≔⟨RGB\; :\; 1\; 0\; 0⟩$

$\mathrm{Hue}\left(c\right)$

$0.$

$c≔\mathrm{ColorTools}:-\mathrm{Desaturate}\left(c\,0.5\right)$

$c≔⟨RGB\; :\; 1\; 0.5\; 0.5⟩$

$\mathrm{Hue}\left(c\right)$

$0.$

$\mathrm{Hue}\left(\mathrm{ColorTools}:-\mathrm{Color}\left(''black''\right)\right)$

$Float\left(\mathrm{undefined}\right)$

The ColorTools[Color][Hue] command was introduced in Maple 2017.

For more information on Maple 2017 changes, see Updates in Maple 2017.