Awesome! Let me just get my exacto knife aaaaaand there's 3 pints of blood on the floor...

Does the rose have to be bloomed already??

Nope, just feed the colored water to the plant. See below for videos.

Does this work with cabbage or children?

Your mileage may vary. Here's the top Reddit comment:

OK - it's happening - I promise to report back my results

3 colours - Red, Blue & Green on a cream rose - let's see how long it takes!

Edit: I was going to make some stupid comment like "It's been 30 minutes and nothing's happened!" but, holyshitballs, IT'S HAPPENING!

Edit 2: 80 mins since starting and the colours are really coming through - however, I realise I've made a terrible mistake in choosing Blue and Green, because Blue + Cream coloured rose = well, green...shit - I have another, much larger rose on my bush that may be more willing to be part of my experiment tomorrow...

Edit 3: 3 1 / 2 hours since experiment began - in realising the folly of using blue & green, I dumped a few drops of red into the blue shot glass to create purple - it may be too late, but let's see what happens. Last update for the evening - will be back in the morning with more info.

Edit: 4: 13 hours later and it looks like she's gone from gulping to sipping - but there's still some definite colour there - the purple is coming through, too, which is good...

Edit 5: Gold? Shit! I was doing this for some fun, but now I'm profiting from it I expect Peter van de Werken's lawyers to be banging down my door demanding payment for a pretty simple process...I may have to go into hiding (ps, thanks /u/ReKch42 :) )

Edit 6: 24 hours later - I think we can all agree that I nailed it - ok, so maybe not - she's going limp, lacking food colouring is dripping down my hands - cat is getting too curious for me to carry on the experiment without having my floors permanently dyed. Thanks for all the support - grab your things - you don't have to go home, but you can't stay here...

Can't tell if this is real or a hoax.

On the use of personal pronouns:  "I have another, much larger rose on my bush that may be more willing to be part of my experiment tomorrow..."   Don't.

Well played, Geege. Well played.

Coloring process:

Bonus -- How to make a fluorescent (glow in the dark) flower: