Icon Coloration Upon Transform

The header is transparent, and icon a bright white with transparency to let the background through. When I begin to scroll, I have the theme set to turn into a white bar (you pros are welcome to tell me the proper vocabulary descriptive of these events and items, and I–and I’m sure you–will be eternally grateful) on which, of course, the icon is not very evident. I would like suggestions, or assistance with perhaps substituting another icon after that transformation has occurred (I have a darker icon suitable not for the page as it currently stands, but for the white bar once an event makes it appear).

Regards.