1.创建矩形
注意：法线的方向，摄像机和光源的位置

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[RequireComponent(typeof(MeshFilter))]
[RequireComponent(typeof(MeshRenderer))]
public class GenerateQuad : MonoBehaviour
{
    public float width = 10.0f;  // 矩形的宽度
    public float height = 5.0f;  // 矩形的高度
    public Material mat;
    // Start is called before the first frame update
    void Start()
    {
        Mesh mesh = new Mesh();

        Vector3[] vertices = new Vector3[4]
        {
            new Vector3(-width / 2, -height / 2,10),
            new Vector3(width / 2, -height / 2,10),
            new Vector3(width / 2, height / 2,10),
            new Vector3(-width / 2, height / 2,10)
        };
        mesh.vertices = vertices;

        // 定义三角形的三角形面（索引）
        int[] triangles = new int[6]
        {
            // 顶点顺序为顺时针
            0, 1, 2,
            2, 3, 0
        };
        mesh.triangles = triangles;

        // 设置法线
        Vector3[] normals = new Vector3[4]
        {
            Vector3.forward,
            Vector3.forward,
            Vector3.forward,
            Vector3.forward
        };
        mesh.normals = normals;

        // 设置UV坐标
        Vector2[] uv = new Vector2[4]
        {
            new Vector2(0, 0),
            new Vector2(1, 0),
            new Vector2(1, 1),
            new Vector2(0, 1)
        };
        mesh.uv = uv;

        // 将创建的Mesh赋给MeshFilter组件
        MeshFilter meshFilter = GetComponent<MeshFilter>();
        meshFilter.mesh = mesh;

        // 为MeshRenderer添加材质，这里需要一个使用顶点颜色的Shader
        // 例如，使用"Particles/Additive" Shader
        //Material material = new Material(Shader.Find("Particles/Standard Unlit"));
        GetComponent<MeshRenderer>().material = mat;

        transform.position = new Vector3(0,0,20);
        transform.localScale = new Vector3(5,5,5);
    }

}

2.建材质和shader

Shader "Custom/quadCol"
{
    Properties
    {
        _Color("Main Color",Color) = (1,1,1,1)
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 100

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            float4 _Color;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                fixed4 col = lerp(_Color,fixed4(0, 1, 1, 1),i.uv.x);
                return col;
            }
            ENDCG
        }
    }
}

3.运行效果如下