The issue persists even if I change the colour of the header: it’s always a slightly lighter version of whatever it should be, but once I scroll it’s fine. I’ve tried inspecting to see what the issue is, but I can’t see anything that’s obviously happening to cause this.
It looks that page is set to overlap the header. This shouldn’t be happening with no Meta Slider in place but can you try editing the page, checking the Page Meta Slider box in the right column, disable overlap and save the page. Any difference?