Your question is excellent and it goes much deeper than you may think. Light is a form of energy and therefore travels in the shape of a wave. The distance between individual waves of light is referred to as the wavelength. See the diagram below.

In the entire spectrum of light a very small range represents visible light. Visible light is made up of a range of wavelengths from violet (purple) at 380 nanometers to red at 620-760 nanometers. Imagine 3 feet divided into 1,000,000,000 (1 billion) different segments and this is the size of one nanometer! We can not actually see visible light, but we can see light when it is reflected off any object, like a blade of grass. For example, if you are in a dark room and you turn on a flashlight you can see a beam of light. The reason you can see that light is because the waves of light energy are being reflected by dust particles in the air! That means that you see the light that is reflected by an object, the dust reflects the entire spectrum of light, making it appear white.

This explains how we see color. Each object is made up of matter. The matter in a blade of grass has chemical properties that both absorb and reflect visible light. When an object absorbs light of a particular wavelength you do not see that color. When an object reflects light of a particular color this is what you can see. Plants are green because they contain a molecule called chlorophyll. This molecule absorbs blue and red light and reflects light in the green wavelength; that is why it looks green. That answers your first question.

Now, I bet you could answer your next question....in the absence of light, would you expect to see color? Nope :). What if you only use red light, or blue light? Feel free to let me know your answer...

Great question!The color of grass leaves (usually green) is mostly due to the absorbance of the other wavelengths of visible light by chlorophyll. Therefore, any differences in gene expression (that ultimately determine phenotype) that affect the concentration of chlorophyll in the leaf will, in turn, affect the color of that leaf. My guess is that, in the absence of light, you could still measure chlorophyll concentration, and assume what the color would be. However, you need transmitted (reflected) light in order to perceive color. I don't know, if a tree falls in the forest, and none is around to hear it, does it make a sound? Hope this helps. Keep the questions coming!