Consistently use 'gray' and 'color'
Summary: There are few 'grey' and 'colour' in the Krita messages. It would be good to use 'gray' and 'color' consistently.
Test Plan: none
Reviewers: Krita, woltherav
Reviewed By: Krita, woltherav
Subscribers: woltherav
Tags: Krita
Differential Revision: https://phabricator.kde.org/D20609