Why plants are red or green
It is a myth that iron makes plants turn red. The reason a plant is red; inefficient chlorophyll, rapid growth, lighting and Anthocyanin. Red coloration in all plants is from primitive Anthocyanin pigment. The more red a plant is the more it needs light to compensate for the fact that it cannot take up red light, because it is red, so it needs more green, yellow and blue light. The reason being; plants are most sensitive to red light and it is very important for photosynthesis. A plant that turns from green to red under high lighting is from the fact that it is growing so fast that the chlorophyll production cannot keep up with the rapid growth. So what you see instead is red Anthocyanin.
You can turn a plant red by reducing Nitrogen and increasing Phosphorus.
This is because Nitrogen is a key element in chlorophyll production. When botanists test to see if a plant is low in Nitrogen they measure the chlorophyll levels in the leaves. Phosphorus promotes rapid growth of new leaves. So if your plant is growing super fast the chlorophyll cannot keep up and if you are inhibiting its growth by not adding Nitrogen it will not produce chlorophyll. Therefore your plant will turn super red. That is the way you do it through human manipulation.
A plant that is naturally red is because it produces inefficient chlorophyll, so more light is needed. Some plants that are green need high lighting because they produce less chlorophyll. That is why they are a light pale green.
The interesting thing is that when a green plant turns red, the plant, in a way, seems to be compensating for the fact that it doesn't need any more light so it produces less chlorophyll and turns red and puts on its “sun-screen”. Also it is because the plant is growing so fast that it cannot produce enough chlorophyll. In reality it is both reasons, but the relationship is so closely connected it can be confusing. Now this is done because plants are most sensitive to the red wavelength and this is most important in photosynthesis. So plants have evolved to produce Anthocyanin which so happens to be red to block excessive photosynthesis from taking place. The plant turns red to block excessive light and from rapid growth simultaneously.
This relates to why tree leaves become red in the fall. The tree has evolved to conserve energy for Winter and needs to drop its leaves as to not waste energy photosynthesizing and producing chlorophyll. All of this takes a great amount of energy, so the first thing to do is shut down chlorophyll production to stop photosynthesis. What remains is Anthocyanin. Now Anthocyanin does allow the tree to continue photosynthesizing, but the weak indirect sun light in the Northern Hemisphere during Autumn months and the primitive inefficient nature of Anthocyanin will not allow for proper production and the leaves fall and the tree goes into hibernation.
The Sun and tilt of the Earth are of course the cause for deciduous forests in the Northern Hemispheres of the Earth and for the seasons. I am explaining, in simple language, how plants have evolved in regards to affects from the Sun and tilt. If they be from indirect PAR (photosynthetically active radiation) or the weather patterns, such as Winter.