Foliage Vertex Color
Vertex color can be used to add fake lighting to flat foliage meshes to create the illusion of a softly-lit foliage volume.
Foliage meshes in game are usually made using flat planes with a transparent texture containing a bunch of leaves (or grass blades). These textures are used instead of modeling each leaf or blade of grass discretely, because the complex geometry renders slowly and also greatly increases the memory cost for the model.
But flat planes generally do not shade very well when lit, so foliage models are often rendered in-game without any lighting (see Emissive). If lighting is an option however, editing the vertex normals is one way to get better lighting.