How to change the color of a mesh?

     Mesh m = new Mesh();
     m.vertices = poss;
     m.uv = uvs;
     m.triangles = new int[] { 0, 1, 2, 1, 2, 3 };
     m.colors = new Color[] {natural, natural, natural, natural };
     m.RecalculateNormals();

The mesh is then added to a GameObject with the components MeshRenderer, MeshFilter and MeshCollider.

The mesh is generated in the position and form I want, but the problem is the color: It's supposed to be green, but it becomes the default pink color.

I have also tried renderer.material.color = Color.green;, but then only 1 triangle (?, one of the two diagonals) of the square is colored. How do I color the other one?

I understand that the reason my vertex colors aren't working is because of my shader: How do I change the Shader?

Why didn't my second try with coloring the material work?