Conditional header text color depending on background image lightness

Hi,

I’m using a full screen black and white photo slider on my homepage. Therefore I’m also using the Enfold transparent header functionality.
However, I’d like to make my header font and logo color dynamic in order to be readable: when a photo in my slider is “light”, I’d like the transparent header to have a dark font and logo, and vice versa.

I know this can be done with specific functions in sass but don’t know how to do it in css.

Can you help?

Thanks in advance
Robin