Create a mesh of 2 triangles, with texture coordinates

triangles3.cs

// Creating Mesh data in Unity
// This script should be attached to a GameObject that has a MeshFilter and MeshRenderer. 
// The script will replace the MeshFilter's geometry by 2 triangles with texture coordinates.
// The vertex coords & tex coords are public variables so they can be manipulated from the Unity inspector.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class triangles3 : MonoBehaviour {

  public Vector3 v0 = new Vector3(-1f,0f,0f);
  public Vector3 v1 = new Vector3(0f,1f,0f);
  public Vector3 v2 = new Vector3(1f,0f,0f);
  public Vector3 v3 = new Vector3(0f,-1f,0f);
  public Vector2 uv0 = new Vector2(0f,0f);
  public Vector2 uv1 = new Vector2(0f,1f);
  public Vector2 uv2 = new Vector2(1f,1f);
  public Vector2 uv3 = new Vector2(1f,0f);

  void Start ()
    {
    Vector3[] newVertices = { v0, v1, v2, v3 };
    Color[] newColors = { Color.red, Color.green, Color.blue, Color.yellow };
    Vector2[] newUV = { uv0, uv1, uv2, uv3 };
    int[] newTriangles = { 0,1,2, 0,2,3 };
    Mesh m = GetComponent<MeshFilter>().mesh;
    m.vertices = newVertices;
    m.colors = newColors;
    m.uv = newUV;
    m.triangles = newTriangles;
    m.RecalculateNormals();
    }
    
  void Update ()
    {
    Mesh m = GetComponent<MeshFilter>().mesh;
    Vector3[] verts = { v0, v1, v2, v3 };
    Vector2[] newUV = { uv0, uv1, uv2, uv3 };
    m.vertices = verts;
    m.uv = newUV;
    m.RecalculateNormals();
    }
}