Stefan,

You could accomplish this with a draw filter.  A quick implementation, with the amazing ability to make your eyes bleed when used on a button, would be the following:

 private class ButtonDrawFilter : IUIElementDrawFilter
{
    public bool DrawElement(DrawPhase drawPhase, ref UIElementDrawParams drawParams)
    {
        Rectangle elemRect = drawParams.Element.RectInsideBorders;
        Rectangle topRect = new Rectangle(elemRect.Location, new Size(elemRect.Width, elemRect.Height / 2));
        Rectangle bottomRect = new Rectangle(new Point(topRect.Left, topRect.Bottom), topRect.Size);

        using (SolidBrush brush1 = new SolidBrush(Color.Red))
        {
            drawParams.Graphics.FillRectangle(brush1, topRect);
        }

        using (SolidBrush brush2 = new SolidBrush(Color.Green))
        {
            drawParams.Graphics.FillRectangle(brush2, bottomRect);
        }

        return true;
    }

    public DrawPhase GetPhasesToFilter(ref UIElementDrawParams drawParams)
    {
        if (drawParams.Element is UltraButtonUIElement)
            return DrawPhase.BeforeDrawBackColor;

        return DrawPhase.None;
    }
}

You would assign a new instance of this class to the DrawFilter property of the UltraButton.  You will likely also have to disable the UseOSThemes property as well.

-Matt