Hue/Saturation on grayscale-range colours

[Alberto Luaces]

Hi,

When I wanted to change a colour (for example, red to green) I always used
the Hue/Saturation trick. But days ago, I realized this trick won't work on
colours that belong to the grayscale range (eg: I can't change the colour
of a <white> t-shirt with this tool). Does anyone know how to transform
this range of colours? Thanks in advance.

Alberto