WORDS, WORDS, WORDS
The best way to learn new slang expressions is to listen to how and where they are being used. The most important thing for a non-native English speaker is to UNDERSTAND slang expressions, not necessarily to use them. If you do try to use a slang expression, use it in the SAME situation in which you heard it being used and do not try to apply it to other situations. This is how slang expressions are often used incorrectly.
Here are some slang expression related to color:
The grass is always greener…
Out of the blue
Once in a blue moon
See you if you can find out the meanings of these slang expressions and in what situations you might use them. Dr. English will provide the answers in the next edition of WORDS, WORDS, WORDS