Линия с изменением цвета

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        DoubleBuffered = true;
    }
    private Point startPoint, endPoint;
    private bool drawMode;
 
    protected override void OnMouseDown(MouseEventArgs e)
    {
        base.OnMouseDown(e);
        startPoint = e.Location;
        drawMode = true;
    }
 
    protected override void OnMouseUp(MouseEventArgs e)
    {
        base.OnMouseUp(e);
        drawMode = false;
    }
 
    protected override void OnMouseMove(MouseEventArgs e)
    {
        base.OnMouseMove(e);
        if (e.Button == MouseButtons.Left)
        {
            endPoint = e.Location;
            Invalidate();
        }
    }
 
    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        if (!drawMode) return;
        e.Graphics.SmoothingMode = SmoothingMode.HighQuality;
        using (var brush = new LinearGradientBrush(startPoint, endPoint, Color.Red, Color.Blue))
        using (var pen = new Pen(brush, 3))
        {
            e.Graphics.DrawLine(pen, startPoint, endPoint);
        }
    }
}