If you can get a point on the slider you could use.
Color color = robot.getPixelColor((int)coord.getX(), (int)coord.getX())
The pseudo code would look like
Robot robot = new Robot();
int xValue = slider.getLocationOnScreen().x + (int)(slider.getValue()/(double)(slider.getMaximum()-slider.getMinimum()) * slider.getWidth() +.5);//Calculates distance of the sliders' position).
int yValue = slider.getLocationOnScreen().y + slider.getHeight()/2;//Calculates a y-value to extract the pixel from.
Color color = robot.getPixelColor(xValue,yValue);