I'm completely sure I am overlooking something but can someone assist ? Which color parameter do I change ?

DO you mean select or put in focus?
 
If you mean in focus you can't. The in focus track now has a highlighted name field, that ties in with the colour changes.
 
If it's selection you mean the track header changes colour just left of the name field. That colour can be changed in Preferences--->Customization--->Colours

Do you mean hi-light a track? That is been dropped in favor of the name label being hi-lighted 

Yes, I meant hi light a track.

It's gone, eh ?

:(

Thx everybody for responding.

No not gone just been replace with a bright track name background.  

That's gotta feel a bit different... Get used to it quickly?

Is there any color differential between track and lanes?

Thanks...

Keni

This only happens if you have the "tracks follow bus" option. If you don't use that, then the selected track background "lightens" as in X2.

Not so much Keni. I was never that happy with how the X series did hi-lighting anyway. Now there is a strong contrast difference with the bright background and the dark background of unselected tracks and can be seen quickly. It is an improvement.  
 
Take lanes; not that I can see but I have not examined them for how well the track colors work with them yet. In a week or so I will maybe have some more info on it. I have a fellow I'm working with on the weekends that is using a lot of take lanes.  

Nah, lanes are the same color as the track that they belong to with a slight indent as per X2.   It would be nice to give lanes a slight color variation, but that would require another preference or color picker.   It might be nice, yes, but not all that critical IMO.  
 
Now the new comping workflow, OTOH, that makes a difference!
 

I use a control surface and, when I touch a fader, it would be nice to have that track header change colors so you know where you are at a glance. 8.3 did this (and was color configurable) and, I miss it.
Just kinda seems like customization/choice really took a back seat between the Sonar 8 series and the X series.