na zacatek tridy

        Pen p;
        Brush b;

na konec konstruktoru

        p = new Pen (panel1.BackColor);
        b = new SolidBrush (panel1.BackColor);

na konec newColorMenuItem_Click

        p = new Pen(t.BackColor);

upravit pictureBox_MouseMove

                /*
                Brush b = new LinearGradientBrush(new Point(0, 0),
                                                  new Point(100, 100),
                                                  Color.Red,
                                                  Color.Blue);
                */
                g.FillEllipse (b, X0, Y0, X1 - X0, Y1 - Y0);
                g.DrawEllipse (p, X0, Y0, X1 - X0, Y1 - Y0);

upravit panel1_MouseDown

        private void panel1_MouseDown (object sender, MouseEventArgs e)
        {
            // Panel t = (Panel) sender;
            // Panel t = sender as Panel;
            Control t = (Control) sender;
 
            if (e.Button == MouseButtons.Middle)
            {
                colorDialog1.Color = t.BackColor;
                if (colorDialog1.ShowDialog() == DialogResult.OK)
                {
                    t.BackColor = colorDialog1.Color;
                    p = new Pen (t.BackColor);
                }
            }
            else if (e.Button == MouseButtons.Left)
            {
                p = new Pen (t.BackColor);
            }
            else if (e.Button == MouseButtons.Right)
            {
                b = new SolidBrush (t.BackColor);
            }
        }

Cely soubor

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
 
namespace malovani
{
    public partial class Form1 : Form
    {
        private Pen p;
        private Brush b;
        private int cnt = 1;
 
        public Form1()
        {
            InitializeComponent();
            pictureBox1_SizeChanged (null, null);
            p = new Pen(panel1.BackColor);
            b = new SolidBrush (panel1.BackColor);
        }
 
        private void panel1_Click(object sender, EventArgs e)
        {
            // p = new Pen (panel1.BackColor);
            p = new Pen((sender as Panel).BackColor);
        }
 
        private void panel1_MouseDown (object sender, MouseEventArgs e)
        {
            // Panel t = (Panel) sender;
            // Panel t = sender as Panel;
            Control t = (Control) sender;
 
            if (e.Button == MouseButtons.Middle)
            {
                colorDialog1.Color = t.BackColor;
                if (colorDialog1.ShowDialog() == DialogResult.OK)
                {
                    t.BackColor = colorDialog1.Color;
                    p = new Pen (t.BackColor);
                }
            }
            else if (e.Button == MouseButtons.Left)
            {
                p = new Pen (t.BackColor);
            }
            else if (e.Button == MouseButtons.Right)
            {
                b = new SolidBrush (t.BackColor);
            }
        }
 
        private void newColorToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Panel t = new Panel();
            t.Parent = toolPanel;
            t.BackColor = Color.CornflowerBlue;
 
            t.Width = panel1.Width;
            t.Height = panel1.Height;
            t.Top = panel1.Top;
            cnt++;
            t.Left = cnt * panel1.Left + (cnt - 1) * panel1.Width;
 
            t.MouseDown += panel1_MouseDown;
            p = new Pen (t.BackColor);
        }
 
        private void pictureBox1_SizeChanged(object sender, EventArgs e)
        {
            Image old = pictureBox1.Image;
 
            int w = pictureBox1.Width;
            int h = pictureBox1.Height;
 
            if (old != null)
            {
                w = Math.Max (w, old.Width);
                h = Math.Max (h, old.Height);
            }
 
            pictureBox1.Image = new Bitmap (w, h);
            Graphics g = Graphics.FromImage(pictureBox1.Image);
            Brush b = new SolidBrush(Color.White);
            g.FillRectangle (b, 0, 0, w, h);
 
            if (old != null)
               g.DrawImage (old, 0, 0);
        }
 
        private void testToolStripMenuItem_Click(object sender, EventArgs e)
        {
            Graphics g = Graphics.FromImage (pictureBox1.Image);
            Pen p = new Pen (Color.Red);
            g.DrawLine (p, 10, 10, 100, 100);
            pictureBox1.Invalidate();
        }
 
        private int X0, Y0;
        private bool s = false;
        private Bitmap save;
 
        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            X0 = e.X;
            Y0 = e.Y;
            s = true;
            save = new Bitmap (pictureBox1.Image);
        }
 
        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (s)
            {
                Graphics g = Graphics.FromImage(pictureBox1.Image);
                g.DrawImage (save, 0, 0);
                // Pen p = new Pen(Color.Red);
                // g.DrawLine (p, X0, Y0, e.X, e.Y);
                int X1 = X0;
                int Y1 = Y0;
                int X2 = e.X;
                int Y2 = e.Y;
                if (X2 < X1) { int t = X1; X1 = X2; X2 = t; }
                if (Y2 < Y1) { int t = Y1; Y1 = Y2; Y2 = t; }
 
                /*
                Brush b = new SolidBrush (Color.Yellow);
                // using System.Drawing.Drawing2D;
                b = new LinearGradientBrush (new Point (0, 0),
                                             new Point (100, 100),
                                             Color.Red,
                                             Color.Blue);
                g.FillRectangle(b, X1, Y1, X2 - X1, Y2 - Y1);
                */
                g.FillEllipse (b, X1, Y1, X2 - X1, Y2 - Y1);
 
 
                // g.DrawRectangle(p, X1, Y1, X2 - X1, Y2 - Y1);
                g.DrawEllipse(p, X1, Y1, X2 - X1, Y2 - Y1);
 
                pictureBox1.Invalidate();
            }
        }
 
        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            s = false;
        }
 
 
 
    }
}