Well it actually worked after tip from Noco Dave at tcp.co forum he wrote:
The CBBg and CBFg midtones have been rigged, to the “Amount” widget. If your not already, this is where you need to change them.
If you look at the output slider for the widget you’ll see the 4 little blue circles tagged to the slider. If you try to change the colours when the slider is in-between the tags you won’t be able to.
You need click on a tag to select it and change the value for that position on the slider, for each of the tags. Then the changes will stick.
But I had to use “save as” command and give the effect a new name. Trying to save the changes into the copy that opens up in motion from within FCPX did not take the changes.