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.click-here
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...