Autum, QQ, kaexin,
I don't think the second question is very fair in any case. There could be cultural context. Besides, normally nobody uses the other colour related idioms on facial expression. The only one I've seen in use is the "as white as snow". It is a bit funny to use the rest. Students can put as black as night because culturally, or at in Sg, we say people's face turn black when they are angry. I think they could have use "he is so frightened, his face turned as white as snow". That would have been obvious. It look like someone is trying to hard to change the questions. :/