I will assume that it has to do with the absorbtion of energy through the light spectrum. Because of the clorophyll in the plants which are green the parts won’t accept green light because they are already green. It has to do with some complex molecular things that I don’t want to get into on why it only absorbs the other colors.

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u r right that green is the least effective color.to ans "why" we should know which colors are mst effective and "why"
photosynthesis can be divided into two phases "light reactions" and dark reaction.light reaction utilizes light energy to form energy rich compounds(ATP and NADPH (1NADPH=3ATP) this is utilized in the dark reaction phase to synthesize starch( the ultimate product)
light reaction takes place in thylakoid membrane of the chloroplast where it has specific chl a rich reaction centres named PS1(or P700) and PSII(or P680) ,both having ability to absorb red and blue light respectivly with maximun wavelenght ranges of 680nm and 700nm.after absorbing light energies these reaction centres run electron tranport chain and proton transport chain with ultimate product of NADPH and ATP respectivly.
so you can see here that there is no chance for anyother color light wavelength wheter green or others.green is rather reflected back thatswhy chl molecule appear green.i hope this helps u .
so we can see here that there is no chance of absorption of anyother light wavelength except blue and red.

Chlorophyll is the molecule that traps this ‘most elusive of all powers’ – and is called a photoreceptor. It is found in the chloroplasts of green plants, and is what makes green plants, green. The basic structure of a chlorophyll molecule is a porphyrin ring, co-ordinated to a central atom. This is very similar in structure to the heme group found in hemoglobin, except that in heme the central atom is iron, whereas in chlorophyll it is magnesium.

There are actually 2 types of chlorophyll, named a and b. They differ only slightly, in the composition of a sidechain (in a it is -CH3, in b it is CHO). Both of these two chlorophylls are very effective photoreceptors because they contain a network of alternating single and double bonds, and the orbitals can delocalise stabilising the structure. Such delocalised polyenes have very strong absorption bands in the visible regions of the spectrum, allowing the plant to absorb the energy from sunlight.

The different sidegroups in the 2 chlorophylls ‘tune’ the absorption spectrum to slightly different wavelengths, so that light that is not significantly absorbed by chlorophyll a, at, say, 460nm, will instead be captured by chlorophyll b, which absorbs strongly at that wavelength. Thus these two kinds of chlorophyll complement each other in absorbing sunlight. Plants can obtain all their energy requirements from the blue and red parts of the spectrum, however, there is still a large spectral region, between 500-600nm, where very little light is absorbed. This light is in the green region of the spectrum, and since it is reflected, this is the reason plants appear green. Chlorophyll absorbs so strongly that it can mask other less intense colours. Some of these more delicate colours (from molecules such as carotene and quercetin) are revealed when the chlorophyll molecule decays in the Autumn, and the woodlands turn red, orange, and golden brown. Chlorophyll can also be damaged when vegetation is cooked, since the central Mg atom is replaced by hydrogen ions. This affects the energy levels within the molecule, causing its absorbance spectrum to alter. Thus cooked leaves change colour – often becoming a paler, insipid yellowy green.

I’ve already wrote in this forum about in this topic. U can search for that!

Thanxs!

Dip Jyoti Chakraborty