Question

Why is the grass green and the sky blue?

Answer 1

Answer to why the grass is green:
The simplest answer to why plants are green is that the parts that are green (e.g., leaves and stems) are reflecting green light. Objects appear certain colors to us because they reflect those colors and absorb every other color. 
Answer to why the sky is blue: 
The sky is blue because of something called Rayleigh scattering. For very small particles such as individual molecules, the wavelength of light that is reflected is a function of the particle size and type of molecular bonds between atoms. It turns out that air molecules are just the right size and the bonds vibrate in such a way that they scatter blue light the most. This causes the sky to be overwhelmed with blue light. 
When you watch the sky turn different colors at sunset, you are watching light become increasingly scattered by more atmosphere and larger particles (dust) as the sun descends toward the horizon. 
On cloudy or foggy days, the water droplets in the atmosphere both absorb the light and scatter all wavelengths equally, causing a grey or white sky.