[Tig] color vision - contrast affects saturation

[Jack Kelly]

Tim Sassoon wrote:

> color lives in the midtones, but in the
> absence of color, we want more dramatic
> tonal separation.

Apologies for hijacking this thread and asking what is probably quite a
beginners... But this question has been nagging me for ages and this seems
like a good time to ask!

I've often wondered why tonal manipulations can affect colour saturation.
For example, take a picture of a sunset... subtle tweaks to the RGB curve
can have quite a profound effect on the saturation of the sky.  For example,
I often find myself using curves to try to make the clouds a little more
contrasty and this can result in the sky changing from a light, watery cyan
to a really saturated cyan.  Why is this?  I guess that saturation and
contrast are linked in as much as desaturation and reduction in contrast
both result in making the image more gray but I can't quite wrap my head
around the relationship between tonality and saturation - any hints would be
most welcome!

Many thanks,
Jack
Dir/Prod/Post
London