There is no single most common colour in nature — the answer depends on whether area coverage, species count, object count or human visual exposure is being measured, and it varies by habitat, geography and season. Green, brown and blue are the colours most often cited, for different reasons.

Green chlorophyll is broken down and reabsorbed by the tree before the leaf falls, unmasking carotenoid and tannin pigments that were in the leaf all along — colours the leaf had all summer, just hidden.
This leaf was scanned flat against a plain background for clarity; the pale scanner background itself was excluded from this palette."Autumn American Sycamore Leaf" by Ninjatacoshell, CC BY-SA 3.0, via Wikimedia Commons
Unlike the sky's or a butterfly wing's structural colour, poppy red comes from anthocyanin pigment in the petals — a genuinely different mechanism producing a superficially similar bright, saturated colour.
Poppy petals are thin and translucent, so their apparent red shifts noticeably with backlighting; this photograph was taken in flat, even daylight."Close up red wild poppy flowers" by Dusan Bicanski, public domain, via Wikimedia CommonsA molecule in the tissue absorbs some wavelengths of light and reflects others — the reflected wavelengths are the colour seen. Chlorophyll, carotenoids and melanin are the most common examples.
Common misconception: Not every vivid natural colour is a pigment — several of the most striking blues and greens in animals are structural colour instead, which looks similar but works completely differently.
Particles or molecules much smaller than the wavelength of light scatter shorter (blue) wavelengths far more than longer (red) ones — the reason a clear daytime sky looks blue and a low sun looks orange or red.
A material absorbs most wavelengths of light and reflects or transmits only a narrow band — deep water, for example, absorbs red light far more readily than blue, which is part of why open ocean water looks blue.
Green is among the most commonly observed colours across terrestrial vegetation, since chlorophyll — the pigment plants use for photosynthesis — reflects green light while absorbing most red and blue wavelengths.
Plants · confidence: high
Brown is widespread across soil, bark, fur and weathered organic material, largely because it results from mixtures of pigments (such as melanin) and mineral content rather than one single compound.
Soil · confidence: medium
Blue and white are commonly perceived across sky and open water, but this is a perceived environmental colour produced by light scattering and absorption, not a pigment present in the sky or the water itself.
Sky and atmosphere · confidence: high
Whether a colour counts as 'common' changes depending on the method: counting land area covered, counting number of species that display a colour, and counting how often a colour is visually encountered can each produce a different answer.
confidence: medium
This is a methodological clarification, not a specific measured finding.
Common
Plants · global, in vegetated regions · chlorophyll-containing plants · area-coverage and species-count estimates cited by botanical sources
Coverage varies enormously between deserts, forests, tundra and urban areas.
Frequency and rarity claims above are scoped to a specific domain, geography or taxonomic group — none are presented as a single universal ranking. See methodology.