ColorWheel.cs

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

[RequireComponent(typeof(MeshFilter))]
[RequireComponent(typeof(MeshRenderer))]
[ExecuteInEditMode]
public class ColorWheel : MonoBehaviour {

	[Range(3, 36)] public int segments = 6;
	[Range(0.0001f, 100)] public float radius = 1;

	#if UNITY_EDITOR
	void Update() {
		if (!Application.isPlaying) SetupMesh();
	}
	#endif

	void SetupMesh () {
		MeshFilter meshFilter = GetComponent<MeshFilter>();

		List<Vector3> verts = new List<Vector3>();
		List<Vector2> uvs = new List<Vector2>();
		List<Vector3> norms = new List<Vector3>();
		List<Color> colors = new List<Color>();
		List<int> tris = new List<int>();

		verts.Add(Vector3.zero);
		uvs.Add(Vector2.one * 0.5f);
		norms.Add(Vector3.back);
		colors.Add(Color.white);

		verts.Add(Vector3.up * radius);
		uvs.Add(new Vector2(0.5f, 1));
		norms.Add(Vector3.back);
		colors.Add(Color.red);

		for (int i = 1; i < segments; i++) {
			float frac = (float)i / (float)segments;
			float angle = Mathf.PI * 2 * frac;
			Vector3 dir = new Vector3(-Mathf.Sin(angle), Mathf.Cos(angle), 0);
			verts.Add(dir * radius);
			uvs.Add(dir * 0.5f + new Vector3(0.5f, 0.5f, 0));
			norms.Add(Vector3.back);
			colors.Add(Color.HSVToRGB(frac, 1, 1));
			tris.AddRange(new int[] {0, i, i+1});
		}
		tris.AddRange(new int[] {0, segments, 1});

		Mesh mesh = new Mesh();
		mesh.name = "ProcMesh";
		mesh.vertices = verts.ToArray();
		mesh.uv = uvs.ToArray();
		mesh.normals = norms.ToArray();
		mesh.colors = colors.ToArray();
		mesh.triangles = tris.ToArray();
		meshFilter.mesh = mesh;
	}
}