Setting brush to point in panel

I am using Windows Forms at new Visual Studio 15 CPT6. I have a class where i set brush to my object example of class:

class Group { private SolidBrush brush; private List points = new List(); private Color color;

    public Group()
    {
        Random randomGen = new Random();
        color = Color.FromArgb(randomGen.Next(0, 220), randomGen.Next(0, 220), randomGen.Next(0, 220));
        brush = new SolidBrush(color);

        Thread.Sleep(10); //This is MUST!
    }

Then in main program i create for example 50 objects of this class and it sets all of them to same color if i dont put Thread.Sleep(10); it works ok if go with step by step debuging but if i just run it it doesnt.

Here is example of calling this methods from main class:

            Graphics graphics = mainPanel.CreateGraphics();

            for (int i = 0; i < points.Count; i++)
            {
                Group newGroup = new Group();
                newGroup.addPointToGroup(points[i]);

                groups.Add(newGroup);

                graphics.FillEllipse(newGroup.getGroupBrush(), points[i].getX(), points[i].getY(), 8, 8);
            }