How do you know if someone is showing their true colours vs just having a bad day?

[ValentineValentini]

A colleague I really like was an arsehole to a junior colleague yesterday. She gave her a task which she knew was beyond her and didn't do anything to help. I've never seen her do anything like that before and it's really making me second guess myself.

Thats her true colours.

Saying something a little snappy or blunt than their normal tone could probably be put down to having a bad day.
Giving a task to someone that exceeded their ability, through lack of thought could be put down as a bad day. But then not helping after that being realised is just plain meanness.