In both, Nativewindow and NEWT, we call:

xswa.colormap = XCreateColormap(dpy,
                                    windowParent,
                                    visual,
                                    AllocNone);

window = XCreateWindow(dpy,
                           windowParent,
                           0, 0, // only a hint, WM most likely will override
                           width, height,
                           0, // border width
                           depth,
                           InputOutput,
                           visual,
                           attrMask,
                           &xswa);

However, at destruction we do not destroy the colormap.

Fix is to call:

XDestroyWindow( display, win );  
XFreeColormap( display, xswa.colormap );