Hello Proveex, thanks for purchasing Thefox!

If you want to do this, you need to install and activate the child theme.

Then go edit the single-portfolio.php ( you can do this from appearance > editor )

Then search for 

overlay_color="'.$mc_bg_color.'"

and replace it with

overlay_color="#ffffff"

change the color code "#ffffff" to the one you want to use instead.

Do not hesitate to contact us if you have more questions.

Have a nice day!

Regards.