Every spring, artists dust off their greens. There are dozens, hundreds of shades of green in nature. Painters can start off with a green like sap green or viridian, and modify it with a touch of yellow and red. Or mix various blues and yellows (I have three of each on my palette, there are dozen more). Then decide how much water to add to make it lighter or darker. Myriad choices to evoke the unfolding colors of the world around us.