// gcc `pkg-config --cflags --libs gtk+-2.0` test.c
#include <gtk/gtk.h>

static void change_color(GtkWidget *button, GtkWidget *bar) {
	GdkColor red;

	red.red = 65535;
	red.green = 0;
	red.blue = 0;

	gtk_widget_modify_bg(button, GTK_STATE_PRELIGHT, &red);
	gtk_widget_modify_bg(bar, GTK_STATE_PRELIGHT, &red);
}

int main (int argc, char **argv) {
	GtkWidget *window;
	GtkWidget *vbox;
	GtkWidget *button;
	GtkWidget *bar;

	gtk_init(&argc, &argv);
	window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
	vbox = gtk_vbox_new(FALSE, 0);
	button = gtk_button_new_with_label("Color red");
	bar = gtk_progress_bar_new();
	gtk_progress_bar_set_fraction(GTK_PROGRESS_BAR(bar), 0.5);

	g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);
	g_signal_connect(button, "clicked", G_CALLBACK(change_color), bar);

	gtk_box_pack_start(GTK_BOX(vbox), button, FALSE, FALSE, 0);
	gtk_box_pack_start(GTK_BOX(vbox), bar, FALSE, FALSE, 0);
	gtk_container_add(GTK_CONTAINER(window), vbox);
	gtk_widget_show_all(window);
	gtk_main();
}