Assigning position-dependant material to GPU-instanced object

Unity Engine
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1

I’m GPU-instancing spheres in a scene in a given, editor-determined random area, I want each instance to get a material depending on its Y axis position, but I have no idea how to do that, I already have 5 different materials created for this and assigned on the editor, but I’m just not being able to figure out how to assign the different material to each GPU-instanced object depending on its Y-axis position.

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

public class ObjData
{
    public Vector3 pos;
    public Vector3 scale;
    public Quaternion rot;

    public Matrix4x4 matrix
    {
        get
        {
            return Matrix4x4.TRS(pos, rot, scale);
        }
    }

    public ObjData(Vector3 pos, Vector3 scale, Quaternion rot)
    {
        this.pos = pos;
        this.scale = scale;
        this.rot = rot;
    }
}

public class GPU : MonoBehaviour
{
    public int instances;
    public Vector3 maxPos;
    public Mesh objMesh;
    public Material Low;
    public Material MidLow;
    public Material Mid;
    public Material MidHigh;
    public Material High;

    Material PlaceHolder;

    private List<List<ObjData>> batches = new List<List<ObjData>>();


    // Start is called before the first frame update
    void Start()
    {
        int batchIndexNum = 0;
        List<ObjData> currBatch = new List<ObjData>();
        for (int i = 0; i < instances; i++)
        {
            AddObj(currBatch, i);
            batchIndexNum++;
            if (batchIndexNum >= 1000)
            {
                batches.Add(currBatch);
                currBatch = BuildNewBatch();
                batchIndexNum = 0;
            }
        }
    }

    // Update is called once per frame
    void Update()
    {
        RenderBatches();
        /*foreach (GameObject batchChild in batches)
        {
            batchChild.transform.position = Vector3.one;
        }*/
    }

    private void AddObj(List<ObjData> batch, int i)
    {
        Vector3 position = new Vector3(Random.Range(-maxPos.x, maxPos.x), Random.Range(-maxPos.y, maxPos.y), Random.Range(-maxPos.z, maxPos.z));
        batch.Add(new ObjData(position, new Vector3(2, 2, 2), Quaternion.identity));
    }

    private List<ObjData> BuildNewBatch()
    {
        return new List<ObjData>();
    }

    private void RenderBatches()
    {
        foreach (var batch in batches)
        {
//HERE IS WHERE I STRUGGLE
            /*if (transform.position.y < -7.0f))
            {
                PlaceHolder = Low;
            }
            if (transform.position.y >= -7.0f && transform.position.y < -5.0f)
            {
                PlaceHolder = MidLow;
            }
            if (transform.position.y >= -5.0f && transform.position.y < 5.0f)
            {
                PlaceHolder = Mid;
            }
            if (transform.position.y >= 5.0f && transform.position.y < 7.0f)
            {
                PlaceHolder = MidHigh;
            }
            if (transform.position.y >= 7.0f)
            {
                PlaceHolder = High;
            }*/
            Graphics.DrawMeshInstanced(objMesh, 0, PlaceHolder, batch.Select((a) => a.matrix).ToList());
        }
    }
}