Code for answer to http://stackoverflow.com/questions/28958106/tooltip-in-picturebox-fillpie-coordinates-c-sharp

CirclesAreFun.cs

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Linq;
using System.Windows.Forms;

namespace ScratchApp
{
    public partial class Form1 : Form
    {

        List<int> values = Enumerable.Range(0, 360).ToList();
        int Rescale(int x) => (int)(((double)x / 360.0) * 255.0);

        public Form1()
        {
            InitializeComponent();

            this.pictureBox1.BorderStyle = BorderStyle.Fixed3D;
            this.pictureBox1.Size = new Size(50, 50);
            this.Size = new Size(450, 450);

            this.DoubleBuffered = true;
            this.Paint += Form1_Paint;
            this.MouseMove += Form1_MouseMove;
        }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.Clear(Color.Black);

            for (int i = 0; i < values.Count; i++)
            {
                Brush b = new SolidBrush(Color.FromArgb(255, Rescale(i), 0, 0));
                e.Graphics.FillPie(b, 0, 0, 400, 400, (float)i, 1.0f);
            }
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            this.pictureBox1.Location = new Point(e.X + 5, e.Y - 5);
            int segment = (int)GetAngle(new Rectangle(0, 0, 400, 400), e.Location);
            this.pictureBox1.BackColor = Color.FromArgb(255, Rescale(segment), 0, 0);
        }

        float GetAngle(Rectangle rect, Point pTo)
        {
            int cx = (rect.Width + rect.X) / 2;
            int cy = (rect.Height + rect.Y) / 2;

            float x = pTo.X - cx;
            float y = (cy - pTo.Y);

            if ((int)x == 0)
            {
                if (y > 0) return 270;
                else return 90;
            }
            else if ((int)y == 0)
            {
                if (x > 0) return 0;
                else return 180;
            }

            float ccwAngle = (float)Math.Atan(Math.Abs(y) / Math.Abs(x));

            if (x > 0 && y > 0)
            {

            }
            else if (x < 0 && y > 0)
            {
                ccwAngle = (float)Math.PI - ccwAngle;
            }
            else if (x < 0 && y < 0)
            {
                ccwAngle = ccwAngle + (float)Math.PI;
            }
            else if (x > 0 && y < 0)
            {
                ccwAngle *= -1f;
            }
            ccwAngle *= (float)(180 / Math.PI);

            while (ccwAngle > 360) ccwAngle -= 360;
            while (ccwAngle < 0) ccwAngle += 360;

            return 360f - ccwAngle;
        }

    }

}