Where to get grass icing tube and colored icing?!
Make grass with Wilton tip #233.
They can be purchased at a Michael's or JoAnne's or your local cake decorating store. You could use a ziploc sandwich bag. Cut the corner out, just big enough for the tip to fit through. Add your frosting and practice squeezing and releasing to make the 'grass look' with the frosting. Pipe onto some waxed paper. You can scrap it off and reuse the frosting so it does not go to waste.
Use food coloring in vanilla icing for your color. If you prefer you could purchase paste color and Michaels as well. You would need one for every color you need. If you are just planning a one time use, I would just purchase some liquid food coloring or get a small set of paste food coloring. Don't spend a lot if you are only doing this once. (It only takes a tiny, tiny amount of paste color if you use that.) Use the end of a toothpick and use a teeny tiny bit. You can always add more.
I make and sell wedding and birthday cakes.
Super Walmart has several wilton icing colors and tips. The grass tip is #233. If you have a michaels or hobby lobby, that would be a better bet/
look for a cake supply store in your area