private void cmbDevity_SelectedIndexChanged(object sender, EventArgs e)//根据combobox选择
        {
            foreach (Control c in Controls)             //清空颜色
            {
                if (c is Button)
                {
                    c.BackColor = Color.Transparent;
                }
            }
            if (this.cmbDevity.Text.Equals("奇数"))
            {
                foreach (Control c in Controls)         //遍历控件，判断
                {
                    if (c is Button && int.Parse(c.Text) % 2 != 0)
                    {
                        c.BackColor = Color.Yellow;
                    }
                }
            }
            else
            {
                foreach (Control c in Controls)
                {
                    if (c is Button && int.Parse(c.Text) % 2 == 0)
                    {
                        c.BackColor = Color.Yellow;
                    }
                }
            }
        }

------解决方案--------------------
        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
       {
           int index = comboBox1.SelectedIndex;
           switch (index)
           {
               //全选,所有按钮紫色
               case 0:
                   button1.BackColor = Color.Fuchsia;
                   button2.BackColor = Color.Fuchsia;
                   button3.BackColor = Color.Fuchsia;
                   button4.BackColor = Color.Fuchsia;
                   button5.BackColor = Color.Fuchsia;
                   button6.BackColor = Color.Fuchsia;
                   button7.BackColor = Color.Fuchsia;
                   button8.BackColor = Color.Fuchsia;
                   button9.BackColor = Color.Fuchsia;
                   button10.BackColor = Color.Fuchsia;
                   break;//不要跳到下个分支
               case 1://奇数
                   button1.BackColor = Color.Transparent;
                   button2.BackColor = Color.Fuchsia;
                   button3.BackColor = Color.Transparent;
                   button4.BackColor = Color.Fuchsia;
                   button5.BackColor = Color.Transparent;
                   button6.BackColor = Color.Fuchsia;
                   button7.BackColor = Color.Transparent;
                   button8.BackColor = Color.Fuchsia;
                   button9.BackColor = Color.Transparent;
                   button10.BackColor = Color.Fuchsia;
                   break;//不要跳到下个分支
               case 2://偶数
                   button1.BackColor = Color.Fuchsia;
                   button2.BackColor = Color.Transparent;
                   button3.BackColor = Color.Fuchsia;
                   button4.BackColor = Color.Transparent;
                   button5.BackColor = Color.Fuchsia;
                   button6.BackColor = Color.Transparent;
                   button7.BackColor = Color.Fuchsia;
                   button8.BackColor = Color.Transparent;
                   button9.BackColor = Color.Fuchsia;
                   button10.BackColor = Color.Transparent;
                   break;//不要跳到下个分支
               case 3://大数
                   button1.BackColor = Color.Transparent;
                   button2.BackColor = Color.Transparent;
                   button3.BackColor = Color.Transparent;
                   button4.BackColor = Color.Transparent;
                   button5.BackColor = Color.Transparent;
                   button6.BackColor = Color.Fuchsia;
                   button7.BackColor = Color.Fuchsia;