The idiom "the curtains match the carpet" -- also heard the other way around and, in American English, swapping in "drapes" and "rug", respectively; I think I've also heard it with "collar" and "cuffs", too -- is used to question whether someone who has dyed their hair has just paid attention to the hair on their head, or has done a more, let's say, "thorough job".
It is quite an obvious analogy. Especially when carpet/rug serves as relatively common slang for a woman's pubic hair and curtains/drapes are quite a good simile for long hair... My point being that you'd have to have lived quite a sheltered existence to neither have ever heard this, nor be able to figure out its meaning!
Is there a drop in replacement which is more subtle, without being a euphemism (e.g., unlike my "thorough job", above)?