Why green is the color that comes back to your eye
A pigment works by taking in some wavelengths of light and letting others pass or bounce away. Chlorophyll is described as the vivid pigment that imparts the green color to plant leaves [2], and chlorophylls are classed among the main pigments that give foods their green color, alongside anthocyanins for red-purple and carotenoids for yellow-orange [5]. In ordinary terms: the leaf soaks up much of the light, and the green portion is what is left to be seen.
That same pigment is not just decoration. Chlorophyll is described as essential and beneficial for both plants and human health [1], and people take it in by eating green vegetables [2]. So the green we see is a side effect of a molecule the plant relies on.
Why some leaves and fruits are yellow-green instead of deep green
The strength of the green tracks how much chlorophyll the tissue contains. In melon, a mutation in a gene called CmYGP, which encodes a Golden2-like transcription factor (a protein that helps switch other genes on or off), produces a yellow-green plant [1]. When researchers reduced the activity of this gene, chloroplast number and chlorophyll content fell, and leaves and fruits turned yellow-green; when they boosted the gene in tomatoes, leaves and fruits became dark green [1].
The gene is active in green tissues, especially young leaves and fruit peels, and it influences the expression of key genes tied to chloroplast development [1]. That fits a simple picture: pale tissue is not a different kind of green, it is less chlorophyll. The melon work links the color change to a specific genetic switch, though the exact chain from gene to pigment is still being worked out [1].
How growing conditions shift the shade of green
Light exposure during growth appears to matter. Shade-grown matcha tea is described as producing higher levels of chlorophyll when sunlight is limited in the final weeks of growth, giving a richer green color [3]. This is a commercial source rather than a controlled study, so treat it as a claim about a cultivation practice rather than a measured mechanism.
Color is also fragile after harvest. Chlorophylls can be affected by temperature, light, oxygen, or pH, which alters their properties, so food processing aims to limit that deterioration [5]. In other words, the green of a leaf on the plant and the green of a vegetable on a shelf are not guaranteed to be the same.
Why some green things also look blue-green
Green is not always the only pigment in play. Spirulina, a blue-green seaweed, gets its green from chlorophyll and its blue from a separate pigment called phycocyanin [4]. That is a useful reminder that "green plant" usually means chlorophyll dominates, not that chlorophyll is the only colored molecule present.
Sources used in this answer
Melon yellow-green plant (Cmygp) encodes a Golden2-like transcription factor regulating chlorophyll synthesis and chloroplast development
A mutation in the melon CmYGP gene, which encodes a Golden2-like transcription factor, reduces chloroplast number and chlorophyll content and produces yellow-green leaves and fruits, while overexpressing it in tomatoes gives dark-green leaves and fruits.
The Metabolism and Potential Bioactivity of Chlorophyll and Metallo‐chlorophyll Derivatives in the Gastrointestinal Tract
Chlorophyll is the vivid pigment that gives plant leaves their green color and is consumed by humans through green vegetables.
Ozoku Matcha – Premium Shade-Grown Green Tea for Energy & Antioxidants
Shade-grown matcha is claimed to produce higher chlorophyll levels and a richer green color when sunlight is limited in the final weeks of growth.
Spirulina applications - marine plant: panoramic review
Spirulina is blue-green because chlorophyll provides the green color and phycocyanin provides the blue.
Impact of ultrasonication applications on color profile of foods
Chlorophylls are among the dominant food pigments responsible for green color and can be altered by temperature, light, oxygen, or pH.