How do I use color allocation?

One of the nice things about GDK is that it's based on top of Xlib; this is also a problem, especially in the area of color management. If you want to use color in your program (drawing a rectangle or such, your code should look something like this:

  GdkColor *color;
  int width, height;
  GtkWidget *widget;
  GdkGC *gc;

  /* first, create a GC to draw on */
  gc = gdk_gc_new(widget->window);

  /* find proper dimensions for rectangle */
  gdk_window_get_size(widget->window, &width, &height);

  /* the color we want to use */
  color = (GdkColor *)malloc(sizeof(GdkColor));
  /* red, green, and blue are passed values, indicating the RGB triple
   * of the color we want to draw. Note that the values of the RGB components
   * within the GdkColor are taken from 0 to 65535, not 0 to 255.
  color->red = red * (65535/255);
  color->green = green * (65535/255);
  color->blue = blue * (65535/255);
  /* the pixel value indicates the index in the colormap of the color.
   * it is simply a combination of the RGB values we set earlier
  color->pixel = (gulong)(red*65536 + green*256 + blue);

  /* However, the pixel valule is only truly valid on 24-bit (TrueColor)
   * displays. Therefore, this call is required so that GDK and X can
   * give us the closest color available in the colormap
  gdk_color_alloc(gtk_widget_get_colormap(widget), color);

  /* set the foreground to our color */
  gdk_gc_set_foreground(gc, color);
  /* draw the rectangle */
  gdk_draw_rectangle(widget->window, gc, 1, 0, 0, width, height);