Transparent header font color for one menu item

HI,

I needed to create a “book now” button within the header of my site.
I followed a few other support threads that got the button working with the quick CSS option.
this is the css I’m using right now…

li#menu-item-647 {
color: #ffffff;
}

li#menu-item-647>a>.avia-menu-text {
background-color: #3399cc;
padding: 10px;
border-radius: 5px;
color: #ffffff;
}
#menu-item-647 span.avia-menu-fx {
display: none!important;
}
li#menu-item-647:hover>a>.avia-menu-text {
background-color: #003366;
}

However, the text in the button when the header is initially transparent the font is black in the button but when you scroll down it changes to white. I need it to just be white no matter what (ex: link in private content section).