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I think it's blue, but I'm not sure.

Is there a way to find out without simply testing it?

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    $\begingroup$ Hi Because yellow light is a mixture of red and green light, yellow will also reflect red and green light. This means that a yellow object will appear red under red light, and green under green light. source for this is scifun.ed.ac.uk/pages/exhibits/ex-colour-box.html $\endgroup$ – user74893 Apr 8 '15 at 18:12
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    $\begingroup$ Is there only green light? If the only photons available are green photons, then the lemon will appear green or black, depending on the brightness of the light source. That said, testing it is the simplest way $\endgroup$ – Jim Apr 8 '15 at 18:12
  • $\begingroup$ @Jimnosperm you are a nanosecond ahead of me .... go on, test it , it's easy to do $\endgroup$ – user74893 Apr 8 '15 at 18:12
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    $\begingroup$ It's not true that all yellow object reflect red and green. I can easily imagine an object with some sort of coating that reflects only "truly" yellow light, i.e., has a very sharp reflectance peak at 600 nm say, but absorbs light very far from this. In general, I would say the answer to this question depends on the particular properties of the material, and we can't say just "x color under white light is y color under z light." But maybe someone knows the answer for a lemon :) $\endgroup$ – zeldredge Apr 8 '15 at 18:22
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    $\begingroup$ Color is fantastically complicated in really interesting ways--the illuminating light, the reflectance of the object at hand, and the sensitivity of our eyes all enter into it, and the ultimate test is subjective/psychological--remember that stupid dress? Anyway, no, the model of color they seem to advance there seems very over-simplified. $\endgroup$ – zeldredge Apr 8 '15 at 18:39
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Probably green, since lemons are common objects in daily life we tend to observe them in daylight conditions often. Based on their bright yellow color (relative to daylight brightness of e.g. white paper) they probably reflect a large part of the spectrum that gives a yellow color, i.e. from red(~600nm) to green(~540nm) wavelengths.

This means under most green lights the lemon will probably be a slightly darker (less luminous) green than the source light--but still green.

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