When separating colors, it is separating the primary colors.
This means red's hex will be: FF0000
Blue's hex will be: 0000FF
Green's hex will be: 00FF00
This means if you have a hex of 00FBFF and separate the colors, and only keep green, the 00FBFF will become green, because it's still got the G in it.
So when trying to change the color blue, you can't ...
for me its working pretty good. using this since blender beta.
few tips to find out..
1- try this sample file
in my case, it looks like this.
steps i did to check
switch to cycles
turn on cryptomatte (as you have already done )
use cryptomatte nodes in compositor, tested via object and material both.
Setup a few extra luxuries on your camera. Such a Depth of Field, which can sometimes be a deal breaker in photorealism. Set it up and see instant realistic results. Play around with the focal length of the digital camera on blender, see which one suits your scene better. There are tons of little settings that you can adjust to make it more believable, but ...