Painting landscapes and vegetation presents many challenges to an artist. Grass is difficult because it is composed of many small strands; painting large areas of grass is particularly challenging because it is important to correctly shade the area or the grass will look unnatural. In addition, perspective in a grassy area is a challenge. While only practice will enable you to paint realistic grass, there are some general techniques that will help you to paint realistic grass in nearly any medium.
Paint a solid field of colour over the area that you want to be grassy, using the general colour that you want your grass to be; that is, paint in a green area if you are painting green grass, yellow or brown for yellow grass. Grass should be one of the last things that you paint, since it will typically cover other objects in your painting, so be sure that you are satisfied with the other aspects of your painting before you start. (see Reference 1)
Shade the colour field, adding in shadows and areas of light. Do not be too detailed.
Mix several shades of the colour you want to use for the grass, making some light and some dark. This will help the strands stand out more easily.
Select a very bristly brush. Lightly brush your grass colours on your painting, alternating the colours and working from the back to the front. Do not make solid strokes with the brush. The grass in the back needs short strokes and the grass in the front needs longer strokes. If you are painting a large area of grass, the strands in the very back will not show at all. Unless it is windy in your image, make the strands bend slightly to both the left and the right.
Select a thin, fine brush. Use loose strokes to paint some solid individual strands in your grass, focusing most of the strands towards the front. These strands should not be taller than the rest of your grass.
Add a few highlights to the grass using a thin brush and a light colour to suggest bright sunlight or rippling grass.
While grass is typically green, yellow or brown, using other colours adds interest to your grass. For instance, using blue and purple in the grass or shading makes green grass look lush. Allowing the paint to dry between layers helps prevent the paint from mixing together and losing the grass effect. Watercolour or ink washes require more planning than mediums like acrylics or oils; gouache or masking fluid can help you create highlighted strands in a watercolour painting.