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); ... }