Change Header font color when hover

Hi,

I would like to change the font color of the menu to black when I go with the mouse to the header. I am using the following CSS to make the header white when the mouse is on it but as far as I am using white font color, I want to change it to black when the header is white.

#header_main:hover {
background: rgba(255,255,255,1);
}

Furthermore, when scrolling down, the header turns white (from transparent). How can I change the font color for the header after scrolling down?

Thank you in advance!
Best,
Anton