Some icons cannot be colored through the options color pickers. That’s because the icon, is SVG based and as known, SVG images actually have XML code inside them.
Therefore some icons have tags that contain fill and stroke attributes eg:
So basically the only solution left is to manually edit the code. This can be done using IconPress’s built-in code editor. Please read here how to edit the code of an icon: Edit an icon’s code (or colors)