using System.Drawing;
using System.Windows.Forms;

namespace WindowsFormsApplication3
{
    public partial class Form1 : Form
    {
        private readonly Button[] buttons;
        private int current;

        public Form1()
        {
            InitializeComponent();
            buttons = new[]
            {
                button1, button2, button3, button4,
                button5, button6, button7, button8,
                button9, button10, button11, button12
            };
        }

        private void timer1_Tick(object sender, System.EventArgs e)
        {
            if (current >= buttons.Length)
                current = 0;
            var button = buttons[current];
            button.BackColor = button.BackColor == Color.LightGreen ? Color.Yellow : Color.LightGreen;
            current++;
        }
    }
}