The secret to perfect lipstick...

Yep!!

I do miss the Pedants' corner threads.

I'm not sure about this one. For example, you wouldn't say 'the key of perfect something-or-other', would you?

I would say "secret of" but "key to"...

Well, yes - but why? Secret and key are both nouns.

Of, I think, for secret but yes, key to! ?

What about ? The secret to success is.....

Perhaps it should be ' the secret to applying lipstick perfectly '

what is the secret?
Mine always disappears after about 15 minutes.

This secret is brilliant, to my lippy. www.amazon.co.uk/Covergirl-Outlast-Lipcolor-Plum-Pearl/dp/B003BMR3IG/ref=sr_1_18_s_it/260-1333840-7235454?s=beauty&ie=UTF8&qid=1503176023&sr=1-18&keywords=CoverGirl&tag=gransnetforum-21

The lipstick in the picture doesn't look perfect to me, anyway. Far too red.

Yes, it's the key TO, but the secret OF.

The fact that they're both nouns makes no difference.

Why do we 'go home' and not 'go TO home' when we go TO a house or TO the shops? (As I was asked more than once by Arabic speakers when teaching them English.)

Well, we just do! All languages that aren't Esperanto have their little quirks.

"To" implies direction but "of" implies possession.

I would have thought that both are correct, but mean slightly different things.

It seems perfectly obvious to me that something like the word “way” is understood and so the phrase has evolved into “The secret (way) to (achieving) perfect lipstick”