Header backdrop colour.

Site URL: https://www.s1xte6n.com

Hi there,

I have set my header to be transparent with a GIF logo in the centre.
This works fine on desktop mode, but when I go to mobile/tablet mode when I scroll down the transparent header turns to a strange brown colour.

I have attempted to change the brown colour through colour schemes but its still happening.

I will attach screen shots showing you what's happening

Screen shot 1 - normal translucent header with gif logo.
screen shot 2 - weird brown header when I hold "Scroll down"

Thanks in advance!