Unable to create a coloured mesh on a canvas.

I've spent most of today trying to draw between 1 and 4 little red triangles on my main UI canvas (ScreenSpace:Overlay), by using CanvasRenderer and a simple script to designate the triangles. Depending on the shader I can either get invisible triangles (any shader except vertex shader) or pitch black ones (vertex shader). The triangles are in the right place and face the right way but refuse to be coloured. The script that generates the triangles is on the same gameObject as a mesh filter and a CanvasRenderer, and this is the code:

         Mesh msh;
         GetComponent<MeshFilter>().mesh = msh = new Mesh();//
         vertices = new Vector3[max + 1];//outside corners and 0
         Color32[] cols = new Color32[max + 1];
         Color[] cols0 = new Color[max + 1];

         triangles = new int[3 * max];
         int vCount = 0;
         int tCount = 0;
         for (float i = 0; i < max; i++) {//this just sets colours and 0-max (0-4) vertices that form a square.
             cols[Mathf.FloorToInt(i)] = new Color32(1, 1, 1, 1);
             cols0[Mathf.FloorToInt(i)] = new Color(1, 1, 1, 1);

             vertices[vCount++] = 100 * Vector3.up * Mathf.Cos((i / max) * 2 * Mathf.PI) + 100 * Vector3.right * Mathf.Sin((i / max) * 2 * Mathf.PI);
         } 
         vertices[vCount] = Vector3.zero;//final vertex at the centre
         cols[vCount] = new Color32(1, 1, 1, 1);
         cols0[vCount] = new Color(1, 1, 1, 1);
         for (int i = 0; i < max; i++) {

             triangles[tCount++] = i; //clockwise to draw
             triangles[tCount++] = (i + 1) % (max);
             triangles[tCount++] = max;
             
         }
         msh.vertices = vertices;
         msh.triangles = triangles;
         msh.colors32 = cols;
         msh.colors = cols0;//this doesn't help, but at least I tried
         Vector2[] uvs = new Vector2[vertices.Length];//don't think this matters

         for (int i = 0; i < uvs.Length; i++) {
             uvs[i] = new Vector2(vertices[i].x, vertices[i].y);
         }
         msh.uv = uvs;
         msh.RecalculateNormals();//its on canvas, why does it need a normal
         msh.RecalculateTangents();
         transform.GetComponent<CanvasRenderer>().materialCount = 1;
         transform.GetComponent<CanvasRenderer>().SetMaterial(mt, 0);
         transform.GetComponent<CanvasRenderer>().SetColor(new Color32(1, 1, 1, 1));
         transform.GetComponent<CanvasRenderer>().SetMesh(msh);`

Any help would be much appreciated, I've spent hours today making basically no progress on this.