The adjust Saturation Colour is a very useful little node that can, as the name says, change the intensity and colour of what ever COLOUR input it receives.
My question is why is it a function node and not a colour node?
I think it's considered a blue node because it doesn't have the typical red shader layout, in that there are no constant settings within the node itself. It needs an external input to set the saturation shift.
You can find it in the Miscellaneous Category, it has a place holder for now. If you'd like to add your thoughts on how to use it, please have a go.
There are few other anomalies in the functions categories, Colour Shaders menu being another, which I have to admit to like being in there for its convenience to access, especially for PFs and Colour Adjust which I tend to use a lot.
Yes that makes sense come to think of it.
I have just started using it to re-purpose image shaders by effectively de-saturating them the colour of a bit map but keeping the contrast. (and maybe a just a little of the colour)
Plumbing a PF in also adds a lot of variation too.