Determine first the kind of grass you'll be drawing. Grass isn't always green. If you ink your grass a single shade of green, you will have flat, unrealistic looking grass. Take into account the type of grass you want to draw. This could be based on geographical location, or you may simply want to color your grass depending upon the season. This information will determine the ink colors you'll use.
Use a thin tipped brush to outline the blades of grass first. Outline carefully, going over the original drawing as best as you can. You won't be emphasizing every blade of grass. The colors you use later on, as well as the way you apply them, will give your grass its final appearance. Outlining each blade of grass will result in an unnatural appearance. The goal here is to get a general shape to your grass. Use a dark green for the outline, which will help produce definition when you apply other colors.
Avoid using only greens for your grass. That would result in a flat, unrealistic painting. Mix some green with brown and a little white to give the appearance of grass that has varying degrees of life to it. Real grass often has dead areas mixed in with the live grass. Some grass varieties have a gray/blue tint. Look at examples and blend colors to create a more realistic final appearance. Even varying shades of green will help. As you apply the colors, use a paint thinner to keep an even distribution.
Use a cross-hatching technique to create the shape of real grass. In real life, grass doesn't typically stand straight up or all lean in one direction. Cross hatching involves placing criss-crossed lines on your paper. Adding these lines gives your grass texture and a more realistic appearance. Make strokes with your brush that go at a slight angle and with varying degrees of height.
Continue to add in a little brown paint mixed with white to deepen the color variation. If the grass you're painting is dying, lessen the green and increase the brown. You can also use the brown throughout to show the ground beneath the grass, which lends a touch of authenticity to the finished piece.