I don’t know how you are verify the connection state but I suppose that you have some kind of trigger to know when the connection is lost and there you add the line to change color.
And when you set the color the visual style doesn’t change, this is why I can assume once I don’t know your code, but here is a tip.
After you change the color use the ‘Invalidate();’ to rebuild your image, so shuld be sonthing like that:
TabPageName.Invalidate();
Hope this help you.