Grass has green color because it has a pigment like other plants called as chlorophyll. This pigment is found in organelles (little factories) in a plant cell known as chloroplasts which is involved in the process called photosynthesis (a process by which plants make their food). In this process carbon dioxide and water make glucose and oxygen.
Carbon dioxide + water – glucose + oxygen
So just imagine that the leaves will not give oxygen if they do not have the chlorophyll for photosynthesis.
This pigment also absorbs short wavelengths with high energy like the blue color and long wavelengths with low energy like the red color but it does not absorb green light and reflects it back. This green light reaches to our eyes and we see the green color of grass and other plants.
Grass appears more bright green on rainy days because on a dry day photosynthesis cannot happen due to lack water. So reactive ions build up and start damaging photosynthesis apparatus like chlorophyll. That is why grass start appearing yellowish but after rain it immediately restores the ability to perform photosynthesis and again appears green.