Hi Vikrant! You're using dual axes, right? If so, you should be able to edit the color mark of just the inner bar. Look for its tab and edit that one only. You may have dragged the calculated field into the "All" tab.
You should also be able to label the inner bar differently using this method by using its specific label mark.
I am not using dual axis so it's just a single view containing a couple of measures to create the Bar-In-Bar chart.
In this situation, I suggest that you convert your existing view to use dual axis. Just drag the two measures separately onto the columns shelf, right-click the rightmost one, and choose "Dual Axis". Then, you'll be able to edit them independently. You can change the size mark of each one to make one thinner.