A lemon abosorbes all pigments of light that are not yellow reflecting visible waves of yellow back at your eyes.

On what level? At the simplest level, lemons are yellow because they reflect the wavelengths we have defined to be yellow and absorb the light wavelengths that are not yellowish.

Because it absorbs all other colours, and reflects only Yellow....

This is not only a question of science, but a question of metaphysics. The lemon is yellow because of the wavelength of light it reflects. But that is only partially true. How we see color is not fully understood and a number of contrasting theories exist. We perceive color based on the way our mind has learned to interpret the signals from our eyes. The surrounding colors affect our perception of color and the ambient light affects our perception, too. Sometimes we can see colors that "aren't there" because of the way we interpret the signal; that is, we might interpret a color as yellow when the actual wavelength of light at which we are looking is one we would usually call grey. Check out this site and select "Color perception" for a striking color illusion:http://www.echalk.co.uk/amusements/OpticalIllusions/illusions.htm The designer of the illusion is R. Beau Lotto whose book "Why We See What We Do: An Empirical Theory of Vision" explains his studies and theories of color perception. The book: http://www.sinauer.com/detail.php?id=7528 Ultimatley, human experience is outside the realm of science since the scientific method can never prove to anyone else that you taste a bitter flavor, feel heat, or are seeing the color you claim.

Just playing dumb, because you've read all the smart stuff...... But lemons are green until they ripen and then they are yellow, because "you" only see yellow light reflected off them. Grasshopper? What color is a lemon in a dark room? Yellow?

There are three levels to this question: Why does it look yellow? Because it reflects yellow and absorbs other colours. What makes it yellow? Zeta-carotene, other carotenes and other pigments. Why does it want to be yellow? To attract animals to eat the fruit and distribute the seeds along with some fertilizer!