Leaf colors

A green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast. When abundant in the leaf's cells, as during the growing season, the chlorophyll's green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus, the leaves of summer are characteristically green.

 In late summer, with daylight hours shortening and temperatures cooling, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. During this time, the amount of chlorophyll in the leaf begins to decrease. Often, the veins are still green after the tissues between them have almost completely changed color.

CHLOROPHYLL

chlorophyll, any member of the most important class of colors involved in photosynthesis, the process by which light energy is converted to chemical energy through the synthesis of organic composites. 

Chlorophyll is set up in nearly all photosynthetic organisms, including green plants, cyanobacteria, and algae. It absorbs energy from light; this energy is also used to convert carbon dioxide to carbohydrates. Chlorophyll occurs in several distinct forms, chlorophylls a and b are the major types set up in advanced plants and green algae.

FLAVONOIDS

Flavonoids, a group of natural substances with variable phenolic structures, are set up in fruits, vegetables, grains, bark, roots, stems, flowers, tea and wine. These natural products are well known for their advantageous goods on health and efforts are being made to isolate the constituents so called flavonoids. 

flavonoid colors are always present in leaves, but as chlorophyll is broken down in the autumn their colours come to the fore. Xanthophyll, a class of carotenoids, are responsible for the yellows of autumn leaves. One of the major Xanthophyll, lutein, is also the compound that contributes towards the yellow colour of egg yolks.

CAROTENOIDS

Carotenoids are actually long- chain water- repelling colors that are synthesized in the plastids of plant cells. In the sunflower, a common carotenoid, ß- carotene, is produced in the chromoplasts of the beam flowers to produce bright yellow- orange colors. These colors primarily absorb in the blue wavelengths, allowing the longer wavelengths to be scattered and producing the yellow color. In autumn season, the carotenoids are left over in the chloroplasts and revealed from the loss of chlorophyll.

Carotenoids also contribute orange colours. Beta- carotene is one of the most common carotenoids in plants, and absorbs green and blue light energetically, reflecting red and yellow light and causing its orange appearance. It's also responsible for the orange colouration of carrots. Carotenoids in leaves start degrading at the same time as chlorophyll, but they do so at a much slower rate; some departed leaves can still contain measurable quantities.

ANTHOCYANINS

Anthocyanins are colorful water-soluble colorants belonging to the phenolic group. The colors are in glycosylated forms. Anthocyanins responsible for the colors, red, purple, and blue, are in fruits and vegetables. Berries, grapes, and some tropical fruits have high anthocyanins content.

Red to purplish blue-chromatic leafy vegetables, grains, roots, and tubers are the eatable vegetables that contain a high level of anthocyanins. Among the anthocyanin colors, cyanidin-3-glucoside is the major anthocyanin set up in most of the plants. The colored anthocyanin colorants have been traditionally used as a natural food color. The color and stability of these colors are impacted by pH, light, temperature, and structure. In acidic condition, anthocyanins appear as red but turn blue when the pH increases.

Anthocyanin synthesis is kick- started by the onset of autumn. As sugar absorption in the leaves increases, sunshine initiates anthocyanin product. The purpose they serve is not clear; it's suggested that they may play a light-defensive part. It was previously supposed they might delay leaf fall, but this has been undefined.

Reference:

1) https://www.encyclopedie-environnement.org/en/zoom/the-colour-of-leaves/
2) https://en.wikipedia.org/wiki/Autumn_leaf_color
3) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5010251/
4) https://en.wikipedia.org/wiki/Flavonoid
5) https://www.livescience.com/52524-flavonoids.html