My manager is a pretty moody woman. Whenever she feels annoyed or doesn't like what you are doing, she would show her displeasure on her face right away. By reading her face, you would know immediately if she is happy or if you should stay away. If she "wears" a grumpy face, you better learn to behave. In Chinese, if someone has a grumpy look, we say he/she is showing the colour or his/her face. Is there something in English that has the same meaning?
You could say that her face is an
open book : someone or something easily understood or interpreted; something very clear: The child's face is an open book.
There are other expressions like to wear one's heart on their sleeve but this is typically used in reference to somebody's love life and not to their mood swings at work (unless their mood swings are a result of the changes in their love life).
This is a case where there can be an advantage in avoiding single words, and some parts of the English speaking world take particular pride in it (we Irish like to think it's one of our specialities).
So, she had a face like a bulldog chewings wasps, she looks like she's licking piss of a nettle, and she gave me a look that would curdle milk are all more evocative than she looked grumpy, and I haven't even started to be imaginative with it yet.