By default, the keybindings of visual-mode in Spacemacs (and by extension, in Vim) are tailored to the Qwerty keyboard layout. In order to remap the keybindings to be better suited for the Colemak keyboard, I used an external package as described below.

Add the colemak-evil package to additional-packages in the .spacemacs file:

dotspacemacs-additional-packages '(colemak-evil)

Require the package in the user-config function in the .spacemacs file:

(defun dotspacemacs/user-config ()
  ...
  (require 'colemak-evil)
  ...
  )

[1] https://github.com/patbl/colemak-evil#setup, "Colemak Evil" [Accessed: 19-May-2018].