C#

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public interface IChild<TParent>
{
    void SetParent(TParent parent);
}

public class ChildCollection<TChild, TParent> : System.Collections.ObjectModel.Collection<TChild> where TChild : IChild<TParent>
{
    public ChildCollection(TParent parent)
    {
        Parent = parent;
    }

    public TParent Parent { get; }

    protected override void InsertItem(int index, TChild item)
    {
        item.SetParent(Parent);
        base.InsertItem(index, item);
    }

    protected override void SetItem(int index, TChild item)
    {
        Items[index].SetParent(default(TParent));
        item.SetParent(Parent);
        base.SetItem(index, item);
    }

    protected override void RemoveItem(int index)
    {
        Items[index].SetParent(default(TParent));
        base.RemoveItem(index);
    }

    protected override void ClearItems()
    {
        foreach (TChild child in Items)
        {
            child.SetParent(default(TParent));
        }

        base.ClearItems();
    }
}

public class OuterClass
{
    public OuterClass()
    {
        AllInners = new ChildCollection<InnerClass, OuterClass>(this);
    }
    
    public IList<InnerClass> AllInners { get; }
}

public class InnerClass : IChild<OuterClass>, IChild<InnerClass>
{
    public InnerClass()
    {
        ChildInners = new ChildCollection<InnerClass, InnerClass>(this);
    }
    
    public OuterClass MyOuter { get; private set; }
    public InnerClass Parent { get; private set; }

    public IList<InnerClass> ChildInners { get; }

    public string Text { get; set; }
    public int Level { get; set; }
    
    protected virtual void SetParent(OuterClass parent, bool fromCollection)
    {
        if (ReferenceEquals(MyOuter, parent)) return;
        
        var oldOuter = MyOuter;
        if (oldOuter != null)
        {
            MyOuter = null;
            oldOuter.AllInners.Remove(this);
        }
        
        MyOuter = parent;
        
        if (parent != null && !fromCollection)
        {
            parent.AllInners.Add(this);
        }
        
        foreach (var child in ChildInners)
        {
            child.SetParent(parent, false);
        }
    }

    void IChild<OuterClass>.SetParent(OuterClass parent)
    {
        SetParent(parent, true);
    }
    
    protected virtual void SetParent(InnerClass parent)
    {
        if (ReferenceEquals(Parent, parent)) return;
        
        var oldParent = Parent;
        if (oldParent != null)
        {
            Parent = null;
            oldParent.ChildInners.Remove(this);
        }
        
        Parent = parent;
        
        var newOuter = parent != null ? parent.MyOuter : null;
        SetParent(newOuter, false);
    }
    
    void IChild<InnerClass>.SetParent(InnerClass parent)
    {
        SetParent(parent);
    }
}

var outer = new OuterClass();
var root = new InnerClass { Text = "A", Level = 0 };
var child = new InnerClass { Text = "A.1", Level = 1 };
var grandchild = new InnerClass { Text = "A.1.i", Level = 2 };

child.ChildInners.Add(grandchild);
root.ChildInners.Add(child);
outer.AllInners.Add(root);

/*
outer.AllInners == [root, child, grandchild]
root.MyOuter == child.MyOuter == grandchild.MyOuter == outer
root.Parent == null, root.ChildInners == [child]
child.Parent == root, child.ChildInners == [grandchild]
grandchild.Parent == child, grandchild.ChildInners == []
*/

// Changing the structure should also work:
root.ChildInners.Add(grandchild);

/*
outer.AllInners == [root, child, grandchild]
root.MyOuter == child.MyOuter == grandchild.MyOuter == outer
root.Parent == null, root.ChildInners == [child, grandchild]
child.Parent == root, child.ChildInners == []
grandchild.Parent == root, grandchild.ChildInners == []
*/

Do I just need to add something below this: 
Color.FromName(colours[res[0] - '0']);
Color.FromName(colours[res[1] - '0']);
Color.FromName(colours[third != 0 ? third : res.Count() - 2]);

private void button1_Click(object sender, EventArgs e)
        {
            if (comboBox1.Text == "4")
            {
                if (textBox1.Text == "0")
                    MessageBox.Show("Otpornost mora biti veca od nule", "Otpornost", MessageBoxButtons.OK, MessageBoxIcon.Error);
 
              //  var black = Color.Black;
 
 
 
                var colours = new string[] { "Black", "Brown", "Red", "Orange", "Yellow", "Green", "Blue", "Violet", "Grey", "Silver", "Gold" };
                var factorDictionary = new Dictionary<char, int>() { { 'R', 1 }, { 'K', 1000 }, { 'M', 1000000 }, { 'G', 1000000000 } };
                string res = string.Empty;
 
                while (string.IsNullOrEmpty(res))
                {
                  //  MessageBox.Show("Enter resistor value (use R, K, M ,G notation for .): ");
                    res = textBox1.Text;
                }
 
                var lastChar = res.Last();
                var isUnitCorrect = factorDictionary.ContainsKey(lastChar);
                var value = res.Substring(0, res.Length - 1);
                var isValueCorrect = !value.Any(x => !char.IsDigit(x));
 
                if (isUnitCorrect && isValueCorrect)
                {
                    int mul = factorDictionary[lastChar];
                    double val = double.Parse(value) * mul;
                    int third = 0;
 
                    if (val < 1)
                    {
                        val *= 100;
                        third = 9;
                    }
                    else if (val < 10)
                    {
                        val *= 10;
                        third = 10;
                    }
 
                    res = val.ToString();
 
                    if (res.Count() > 11)
                        MessageBox.Show("Invalid value");
                    else
                    {
                        label15.BackColor = Color.FromName(colours[res[0] - '0']);
                        label16.BackColor = Color.FromName(colours[res[1] - '0']);
                        label17.BackColor = Color.FromName(colours[third != 0 ? third : res.Count() - 2]);
                    }
                }
                else
                    MessageBox.Show("Invalid value!");
            }
        }