Change Navigation Bar (Background and Text) Color Post-Scroll

[Vreal]

First pic is the header when it loads normally from simply navigating to the website (www.royaltontravel.com) as it should be, white font, transparent background, and menu box white/transparent...great, however;

Second pic is after the scroll...background goes white and the font goes black. Instead, I want the background to go dark grey (as the background in the rest of the page) and the font to go white.

How do I make the post scroll background grey with white font?? (while keeping the menu button outline/font white with transparent background.)
I've read a bunch of help threads and can't figure this one out. I am using the High Peak - Birdseye 2. Thanks in advance for the assist.