public class Circle
{
    [XmlAttribute]
    public string Color;

    [XmlIgnore]
    public MinMax Radius;

    [XmlAttribute("Radius")]
    [EditorBrowsable(EditorBrowsableState.Never)]
    public string minmax
    {
        get { return Radius.Min + ";" + Radius.Max; }

        set
        {
            var arr = value.Split(';');
            if (Radius == null)
                Radius = new MinMax();
            Radius.Min = int.Parse(arr[0]);
            Radius.Max = int.Parse(arr[1]);
        }
    }
}

public class MinMax
{
    public int Min;
    public int Max;
}