Custom color for Button focus rectangle and cue border.

Make sure the first button is the "one and only" control with TabIndex=0 so it gets the focus, but there shouldn't be a difference between this and being tabbed into.

If you choose to "custom paint that" you can make it minimally invasive by manually swapping out all Button for ButtonEx. Then, with minimal code, you can have a design-time property for the outer focus cue solid border as well as the dotted focus rectangle.

---

class ButtonEx : Button
{
    public Color FocusRectangleColor
    {
        get => _focusRectangleColor;
        set
        {
            if (!Equals(_focusRectangleColor, value))
            {
                _focusRectangleColor = value;
                Refresh();
            }   
        }
    }
    Color _focusRectangleColor = Color.Red;

    public Color FocusCueColor
    {
        get => _focusCueColor;
        set
        {
            if (!Equals(_focusCueColor, value))
            {
                _focusCueColor = value;
                Refresh();
            }
        }
    }
    Color _focusCueColor = default;

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        if (Enabled && Focused && (MouseButtons == MouseButtons.None))
        {
            if (FocusRectangleColor != default)
            {
                using (Pen dottedPen = new Pen(FocusRectangleColor, 2f))
                {
                    dottedPen.DashStyle = System.Drawing.Drawing2D.DashStyle.Dot;
                    Rectangle focusRect = this.ClientRectangle;
                    focusRect.Inflate(-12, -12);
                    e.Graphics.DrawRectangle(dottedPen, focusRect);
                }
            }
            if (FocusCueColor != default)
            {
                using (Pen cuePen = new Pen(FocusCueColor, FlatAppearance.BorderSize))
                {
                    Rectangle focusRect = this.ClientRectangle;
                    focusRect.Inflate(-2, -2);
                    e.Graphics.DrawRectangle(cuePen, focusRect);
                }
            }
        }
    }
}

---

Testing

The code as written should navigate correctly when:

• The [Tab] key is used to navigate.
• The mouse is used to click the button.
• button.Focus() is called programmatically.
• SelectNextControl() is called from the currently focused control.
• ActiveControl is set programmatically.

---

In order to behave:

• The TabOrder of the controls needs to be set correctly
• The TabStop property should be true for controls you want to tab otherwise false.
• The button container (e.g. Form) must be active.
• The button that should have focus must have the lowest tab index.

Testbench

Here's the code I used to test this answer with with the values for TabIndex as shown in VS Menu\View\Tab Order. Clone. Using the combo box, timer modes can be selected to cycle though the three "programmatic" options of navigation from the list above.

public partial class MainForm : Form
{
    enum TimerMode { None, FocusNext, SelectNext, NextActive }
    public MainForm()
    {
        InitializeComponent();
        comboBoxTimers.Items.AddRange(Enum.GetValues(typeof(TimerMode)).OfType<object>().ToArray());
        comboBoxTimers.SelectedIndex = 0;
        comboBoxTimers.SelectionChangeCommitted += async(sender, e) =>
        {
            _timerMode = TimerMode.None;
            await _busy.WaitAsync();
            _timerMode = (TimerMode)(comboBoxTimers.SelectedItem ?? TimerMode.None);
            BeginInvoke(()=>ActiveControl = button1);
            string name;
            switch (_timerMode)
            {
                case TimerMode.FocusNext:
                    do
                    {
                        await Task.Delay(1000);
                        if (!comboBoxTimers.DroppedDown)
                        {
                            name = ActiveControl?.Name ?? string.Empty;
                            switch (name)
                            {
                                case nameof(button1): button2.Focus(); break;
                                case nameof(button2): button3.Focus(); break;
                                default: case nameof(button3): button1.Focus(); break;
                            }
                        }
                    } while (_timerMode == TimerMode.FocusNext);
                    _busy.Release();
                    break;
                case TimerMode.SelectNext:
                    do
                    {
                        await Task.Delay(1000);
                        if (!comboBoxTimers.DroppedDown)
                        {
                            if (ActiveControl != null) SelectNextControl(
                                ctl: ActiveControl,
                                forward: true,
                                tabStopOnly: true,
                                nested: true,
                                wrap: true
                            );
                        }
                    } while (_timerMode == TimerMode.SelectNext);
                    _busy.Release();
                    break;
                case TimerMode.NextActive:
                    do
                    {
                        if (!comboBoxTimers.DroppedDown)
                        {
                            await Task.Delay(1000);
                            name = ActiveControl?.Name ?? string.Empty;
                            switch (name)
                            {
                                case nameof(button1): ActiveControl = button2; break;
                                case nameof(button2): ActiveControl = button3; break;
                                default: case nameof(button3): ActiveControl = button1; break;
                            }
                        }
                    } while (_timerMode == TimerMode.SelectNext);
                    _busy.Release();
                    break;
            }
        };
    }
    TimerMode _timerMode = TimerMode.None;
    SemaphoreSlim _busy = new SemaphoreSlim(1, 1);
}