To change the GC in a style:

1: <?php
2:   $gc = $style->fg_gc[GTK_STATE_NORMAL];
3:   $gc->foreground = $color;
4:   $style->fg_gc[GTK_STATE_NORMAL] = $gc;
5: ?>

Manipulating widget style properties by hand is very cumbersome and is also prone to clash with the theme that the user has chosen for themselves. Instead, try to use resource files when you need to provide control over widgets' appearance.