Why aren't plants black? 🌿

Good thinking. The same question came in my mind 12 years ago, and I decided to study the topic more carefully. I even made experiments which supported my hypothesis. 

First, the Plants are basically just like any factory which produces something. They use Water, CO2 and sunlight to produce sugar and O2. 

So first we need to find what limits this production. In Ocean it surely isn't water, and it isn't sun light either. It's CO2
Notice that when a plant Takes CO2 from air and releases the O2 out, without wind, it would pretty soon be macroscopically surrounded by O2, and the average CO2 amount in air doesn't help anything. The Plant is out of supply of CO2. 
As this Wind stuffy happen's over the time, it's obvious that The Plants should be using the light which provides just enough energy for the process AND is longest present for the process. -> Blue/lila light is the first and last light of the day due to the Refraction. So basically the Plants are green because the Sky is Blue. But it's not that simple. 
Earth saves the heat of the day and radiates it back on the night. And plants use this  "red" ligh too for the same reason. It's availability over time. 

Now, if this hypothesis is true, then providing more CO2 to plants should change their colour. And this actually is true, just look the grass after you have cut it. The part of grass closest to the ground is more yellow, because the humus supplies CO2, this means the average refleced wavelength might have gone from 550 nm to 580 nm.

I have observations (fotos) from nature, where even trees change their colours to more yellow, if there is dying tree providing more CO2 in some windshade place in mountains. 

But the most interesting thing I did was the experiment with Sphagnum; I gave them pure CO2, (In a controlled setup) and here I could see how they changed their colour to yellow, and could visually obsorve that they were growing faster.  Due to other interests, I didn't have the time to make ful scale study with accurate measurements. Bot what I found satisfied my own curiosity. The reason I chose Spaghnum is because it builds a structure, even though every single cell is fully functioning plant. This Structure gives the possibility to have a open structure and efficiently vented gas exchange; to deliver the gas and water directly where the plant can use it.  
The algen have the problem with difficult gas exchange, the O2 must get out of water, and CO2 must get in the water. 

btw. It's commonly known in Finnland that peat moss can change it's colours during the season, from red to yellow to green  though the different coloured plants have given different names; 
https://en.wikipedia.org/wiki/Sphagnum#Taxonomy_and_phylogeny

I personally am convinced that this colour is caused by the local CO2 supply, which can vary ie. because of water level changes etc.