When people first started using flags on ships as a means of identification, the actual design of the flag could be a bit so-so. But the color would tell you whose ship it was.
In war and piracy, sailing under the flag of a supposedly friendly nation could let you get real close before revealing your "true colors", and in that way get an advantage over your intended victim.
This isn't an idiom. Telling a story means to speak and relay a tale - which can be a true one or a false one.
It depends on how you use it. If you mean literal colors, then it's not an idiom. If you say something like "It's all there in black and white," then it's an idiom meaning that something is printed.
He showed his true colors when he did that.
This is a Mexican idiom that means to lie or make false promises.
When someone challenges a false statement you've made or threat you didn't intend to follow through with.
A mistake or false
RFP is not an idiom. It's an abbreviation.
It's not really an idiom. It means "what are you thinking about."
It is not an idiom, it means your nose is itching.
idiom means expression like a page in a book
It's not an idiom. It means the tip of your nostril.
"Sieve" is not an idiom. See the related link.