Why are plants green?

Your question is based on a false premise.

Chlorophyll is not the only pigment used by plants to extract energy, it's not even true of green plants. Green plants use a range of accessory pigments in conjunction with chlorophyll, specifically to harvest light energy and convert it to something useable, they include:
Carotene - an orange pigment
Xanthophyll - a yellow pigment
Phaeophytin a - a gray-brown pigment
Phaeophytin b - a yellow-brown pigment

 

This is beginning to smell like creationist trolling (it also sounds like a 'No True Scottsman' Fallacy).

You're going to have to explain how you define a 'true leaf' because (for example) Poinsettias only have one kind of leaf, and they're red.

 

Retinal was (and still is) the colour it is because the total length of the conjugated double bond system matches the wavelength of a photon with a wavelength of around 400nm, and so Retinal can absorb photons of this wavelength, and in so doing is excited into a higher energy electronic configuration, however because Retinal absorbs in the blue-green part of the spectrum, it appears red-orange to us. It appears red-orange to us, because our eyes see in three colours (red, green, and blue) or in a very few cases four (predominantly women). Our brain then 'adds' the strength of the responses in these three bands and 'calculates' the colour it is seeing.

We see it as Orange, because what we're seeing is mostly red with a little bit of green and no blue.

Attaching the Retinal to other stuff, as it would have to be to be useable by plants, changes (lowers, in this case, I believe) the wavelength of light that is absorbed, by changing the length of the conjugated double bond system, resulting in a slightly different colour.

EG:

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This is why Rhodopsin (what things using Retinal to make food use) is purple.

 

This is a bait and switch - whether or not I think you're a creationist is irrelevant to the discussion.

I posed you a legitimate, relevant, question, "What do you define as a true lead". And you're using the point that I expressed the opinion that your posts were starting to sound like creationist trolling as a distraction to avoid answering it.

 

So then the answer to your question "Why is Retinal purple" is "Because of its physico-chemical properties", and you accept that as an answer.

 

No trolling involved, on my behalf at any rate.

I've already explained that it's not directly relevant, simply an opinion reflecting on the language you've used so far. Move on.

This, is trolling. Not to mention ridiculous. I know exactly why I made the comment, and won't answer is not the same as can't answer. One does not imply the other.

Which would be great if I were only talking about a minority of plants, but I'm not.
EVERY SINGLE PLANT, for example, contains caretenoids of some form, to some extent. The also contain each and every single on of the other pigments I named, in varying degrees and combinations, this is why plants are so many different shades of green.

Your question has been answered, however.
Q: Why are plants green?
A: Because of Chlorophyll.

Q: Why is Cholorphyll green?
A: Because Chlorophyl contains a cyclic conjugated double bond system, which has a magnesium ion bound to it (as a ligand). It's green because the total length of the conjugated system matches the wavelength of a red photon.

Q: Why do modern plants use Chlorophyll for photosynthesis?
A: Perhaps Plants using chlorophyll had to compete with plants using pigments that absorbed in the green portion of the spectrum, and chlorophyll outcompeted them because it was more efficient at converting energy into food.
Perhaps the over abundance of energy in the part of the spectrum is detrimental.
Perhaps the different atmospheric chemistry of the early earth (at the time Autotrophs evolved) scattered and absorbed light differently from what is observed now - resulting in red light being the most widely available light source at ground level.
Perhaps absorbing only low energy radiation reduced early mortality - the early atmosphere may have been a lot more transparent to Higher energy frequencies, which do more damage (this is related to why colours fade with time). This has the advantage of explaining why plants incorporate the auxillary pigments they do - they all absorb strongly in the UV/Blue end of the spectrum, some of them have anti-oxidant properties, and none absorb in the green portion. This has the net effect of protecting Chlorophyl from the damaging high energy radiation (Chlorohyll also absorbs in the Blue/UV part of the spectrum), as well as protecting it (and the rest of the cell) from the effects of damage that is done.
Perhaps a combination of all of the above.
Perhaps none of the above - at this point, all we can really do is speculate, and through that speculation we can't rule out the existence of multiple photosynthetic pathways in the early history of the earth, using a variety of pigments, the only thing we can say for certain is that there was some environmental factor that meant that those organisms using chlorophyl based photosynthesis were more successful than the others.

Q: Why aren't the pigments that plants use black?
A: Because pigments that absorb across the whole electromagnetic spectrum degrade very quickly (related to the penultimate point above).