More green in color, or more inexperienced and naive (as in 'greener than a new recruit').
From 'green' (Old English 'grene') plus comparative suffix '-er.' The 'inexperienced' meaning comes from the color green being associated with youth and newness.
The phrase 'the grass is always greener' comes from the idea that new growth appears brighter and more full of promise—but in nature, grass only looks greener during spring rains, so it's literally a trick of appearance!
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