[ticket/12176] Add newest_user_colour to config array to ensure it exists

PHPBB3-12176
Merge pull request #6 from nickvergessen/ticket/bantu/12176

[ticket/12176] Add newest_user_colour to config array to ensure it exists

* nickvergessen/ticket/bantu/12176:
  [ticket/12176] Add newest_user_colour to config array to ensure it exists
[ticket/12176] Reword comment about config entries.

PHPBB3-12176