If you use the Inspect tool in your browser, you can see the CSS and see if any other selectors are overwriting it.
In the case here, you have the link color set in the page header, so the CSS for that is:
.inside-content-header a
Then your button is:
.custom-button
So the page header CSS is more specific, meaning it will overwrite your CSS.
You can either make your CSS more specific:
.inside-content-header a.custom-button
Or add !important after your link color:
color: #FFF !important;