Try this after the Display() :
sf::Image screen = sf::Image::CopyScreen(App);
Then parse your image and get the color of each pixels with sf::Image::GetPixel(x, y), but it will be horribly slow.
The color of the sf::Shape::Circle function give the color of each vertices of the circle. If you want a global color, create your circle like this :
sf::Shape circle = sf::Shape::Circle(x, y, radius, sf::Color::White);
circle.SetColor(sf::Color::Red);
Then you can get the global color of your circle with GetColor().