Controlling saturation on scene referred data As troy_s mentioned in his answer about CDL node, Saturation is part of ASC CDL, but blender doesn't incorporate it in the CDL node. And 


  to control saturation on scene referred data a custom node group needs
  to be created to operate after the CDL node.


So, I searched about it on internet and found that:


  A fourth parameter “Saturation” is achieved by converting the out.rgb
  data in a Luma and Chroma component. The Chroma Signal is then
  multiplied by the “Saturation” parameter.


I created this nodes and they seems work, but I'm not sure about math, is it the right way to control saturation?
The sample code for the ASC-CDL Saturation component looks like this:

/* compute the fully desaturated grey value */
luma =  rgb[0] * sat_coef_709[0] +
    rgb[1] * sat_coef_709[1] +
    rgb[2] * sat_coef_709[2] ;

/* 
output rgb is a weighted average between the input rgb and 
the fully desaturated grey value with "sat" as the weight...
*/

for (c=0; c&lt;3; c++)
    rgb[c] = rgb[c] * sat + luma * (1.0 - sat);


The coefficients for any given RGB colourspace will differ. The coefficients for REC.709, are RGB 0.2126, 0.7152, 0.0722

Should be easy to sort out.

And yes, as per @cegaton, avoid the horrible use of (the also equally incorrectly labelled) YUV.

Technically, the RGB to BW node should work and honour the luma coefficients in the OpenColorIO configuration as well.