The pages you showed as examples have background set it Visual Composer editor, but Portfolio page uses portfolio.php template.
I’ve added a new class <div class="contback"> in wp-content/themes/royal/portfolio.php file.
You need set background color in custom.css for this class. Add the following code:
How to create custom.css you can watch in this tutorial (it’s for Legenda theme but the process is the same).