GTK3 theme symbolic icon coloring

["J.L.M." <jlyonm gmail com>]

I'm trying to color or recolor a gtk icon. I have tried many things, but nothing has worked so far. I haven't found an example, either. Here's most of what I've tried:

@define-color fg_color rgb(166, 0, 255);

spinner {

    -gtk-icon-source: -gtk-icontheme("process-working-symbolic");

    -gtk-icon-palette: success @fg_color, error @fg_color, warning @fg_color;

}

/*or*/

spinner {

    -gtk-icon-source: url('assets/check-symbolic.symbolic.png');

  -gtk-icon-palette: success @fg_color, error @fg_color, warning @fg_color;

}

/*or*/

spinner {

-gtk-icon-source: -gtk-recolor(url('assets/check-symbolic.symbolic.png'), success blue, error red, warning green);

}

The PNG is a file I got from another theme already installed. It's black normally, but if I try to apply the palette or recolor to it, it shows up as white. The "process-working-symbolic" always displays as white. How do I make either the built-in image or the PNG to use a color other than white?

Thank you for your time.