Q3.2.3: How can I set the colors when highlighting a region?

How can I set the background/foreground colors when highlighting a region?

You can change the face zmacs-region either in your .Xresources:

Emacs.zmacs-region.attributeForeground: firebrick
Emacs.zmacs-region.attributeBackground: lightseagreen

or in your init.el:

(set-face-background 'zmacs-region "red")
(set-face-foreground 'zmacs-region "yellow")