Vertex coloring multiple submeshes different colors

Im trying to improve my games draw calls by using the same material on each submesh and changing its vertex colours instead of making a new material for each colour.

Here is a modified script that is nearly there. However as you can see there is still red where the tracks vertices are in both submeshs.

What can I do to fix this?

 using UnityEngine;
 using System.Collections;
 [ExecuteInEditMode]
 public class updateVerts : MonoBehaviour {
 
     public Color[] newColors;
     
     void Update () {
         Mesh mesh = GetComponent<MeshFilter> ().sharedMesh;
         Color[] colors = new Color[mesh.vertices.Length];
 
         for(int i = 0; i < mesh.subMeshCount; i++) {
             int[] t = mesh.GetTriangles(i);
             for (int j = 0; j < t.Length; j++) {
                 colors[t[j]] = newColors[i];
             }
         }
         mesh.colors = colors;
     }
 }