hit-moodle/moodle-local_onlinejudge — comment on commit aeebf1df89c8bc7b5476f3e8b916c39520d2a220
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By weezel:

@liuyu81 noup, it's the part of the basic UI design: http://www.infoq.com/articles/Colors-UI and http://msdn.microsoft.com/en-us/library/windows/desktop/aa511283.aspx . The latter one states: "Meaning. The meaning of a message can be summarized through color. For example, color is often used to communicate status—where red is a problem or error, yellow is caution or warning, and green is good."