Purple prose is the name given to writing, that's just too flowery, too melodramatic, too overdone. I think perhaps writers who don't have confidence in their own skills write to impress and can run the risk of their prose getting too purple. They're trying too hard and therefore are over-compensating.
Having said all that, there is no ultimate absolute definition. Perhaps one person's purple prose is another person's vivid description.
It's a subjective decision - part of the multi-faceted balancing act writers undertake. A lot would depend on the genre too - you'd get away with a lot more in say, the romance genre, than you would in the thriller genre.
But I think it's safe to say that any sentence including the phrase 'rosy fingers of dawn' would qualify.
As we discuss in metaphors you need vivid description to make the reader visualise what you're describing. But it needs to be restrained at the same time. The goldilocks version: neither too much, nor too little, but just right.
Watch out too for words which are too big or fancy. Sometimes you have to use a very specific word because you need an exactness of meaning. But most often you can use simpler words. Writing isn't about using fancy words; it's about using simple words bravely and honestly.
One place where you have to especially watch out for purple prose is during love scenes. The clinical terms around bodies and love-making are just so, well, clinical. But a lot of the slang words are very crude. So - assuming you write sex scenes rather than just setting it up and then letting the reader imagine the rest - you have to find ways to describe the act without being either clinical or crude.
The problem with this is that you can get very purple. His throbbing manhood, her heaving bosoms ... argh.
You'll have to find your own way to solve that problem - but hopefully I've alerted you to the danger, and during the editing process you can watch out for this and correct it.
Killing Your Darlings
Another clue to whether the writing is overdone is that it draws attention to itself rather than to the story. Every time you think proudly to yourself, That's a lovely phrase, take that as a warning sign.
If the phrase is self-indulgent, and is far more about your own cleverness than it is about the story, then it has to go. This is what is meant by the phrase, 'kill your darlings'.
This is not to say that you have to lose every phrase you're particularly pleased with - no, no, a thousand times no! (Although you may well see that 'advice' elsewhere - just ignore it if you do.)
If the phrase serves the story, it stays. Otherwise it goes. It's very simple. Simple, perhaps, but not always easy: it is hard, I totally acknowledge, to kill your darlings. One trick is to have a file on your computer where you save them; that way they neither clutter up your story, nor are lost forever.