Flag - fix purple flag

Summary: purple != violet, and in our CSS we call these things by the fanciest of terms. Fixes T5995.

Test Plan: flagged something purple and saw that the "remove purple flag" flag was indeed purple. quickly tested other colors and they all seem good too.

Reviewers: epriestley, chad

Reviewed By: chad

Subscribers: epriestley, Korvin

Maniphest Tasks: T5995

Differential Revision: https://secure.phabricator.com/D10389