[gtkmm] Gdk::Color - can't understand

[Igor Gorbounov <igorbounov voronezh serw ru>]

Explain me, why values in Gdk::Color::set_rgb(red, green, blue) must be 
2-byte
values? I mean, that to produce a 0x3fafc0 color I have to write it as
set_rgb(0x3fff, 0xafff, 0xc0ff), adding one extra byte to every color 
component.
And next, from reading gtkmm and gtk documentation I nevertheless can't
understand whether I really do need colormap or just can easily live on
with Gdk::Drawable::set_rgb_foreground() to draw my rectangles and lines
on a drawingarea.
Explain me, please (hope, there is somebody, who knows the answer).
   Regards,
      Igor Gorbounov