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private void Form1_Load(object sender, EventArgs e)
{
    comboBox1.Items.AddRange(Enum.GetNames(typeof(KnownColor)));
    comboBox1.DrawMode = DrawMode.OwnerDrawFixed;

    //comboBox1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
    //comboBox1.AutoCompleteSource = AutoCompleteSource.ListItems;
}

private void comboBox1_DrawItem(object sender, DrawItemEventArgs e)
{
    if (e.Index < 0)
        return;

    string colorName = comboBox1.Items[e.Index].ToString();
    var color = Color.FromKnownColor((KnownColor)Enum.Parse(typeof(KnownColor), colorName, true));
    Brush brush = new SolidBrush(color);

    var simpleContrastingColor = Color.FromArgb((byte)~color.R, (byte)~color.G, (byte)~color.B);  // Hintergrundfarbe billiges invertieren 
    e.Graphics.FillRectangle(new SolidBrush(simpleContrastingColor), e.Bounds);
    e.Graphics.DrawString(colorName, e.Font, brush, (RectangleF)e.Bounds);
}