Mesh.colors  

// Sets the vertex color to be red at the y=0 and green at y=1.
// (Note that most builtin shaders don't display vertex colors, you can
// use eg. a particle shader to see vertex colors)
function Start () {
var mesh : Mesh = GetComponent(MeshFilter).mesh;
var vertices = mesh.vertices;
var colors = new Color[vertices.Length];

for (var i=0;i<vertices.Length;i++) {
colors[i] = Color.Lerp(Color.red, Color.green, vertices[i].y);
}
mesh.colors = colors;
}