If you click the Edit button and then color button and set a color, darvas box isn't painted with the specified color. The problem occurs,too when you try to change the darvas color by code. To reproduce the problem you can see the All Features\Welcome !\Chart styles\Financial\Darvas demo example or use below code:

 public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            tChart1.Aspect.View3D = false; 
            Steema.TeeChart.Styles.Darvas darvas = new Steema.TeeChart.Styles.Darvas(tChart1.Chart);
            darvas.FillSampleValues(10);
            darvas.Color = Color.Yellow; 
        }

        private void button1_Click(object sender, EventArgs e)
        {
            tChart1.ShowEditor();
        }