The header file graphics.h contains getpixel() function which returns the color of pixel present at location (x, y).
int getpixel(int x, int y);
Note : By default the screen is BLACK, therefore color of pixel at (0,0) is BLACK.
Below is the implementation of getpixel() function.
Explanation : As nothing is drawn anything on screen and by default screen is BLACK, therefore color of pixel at (0, 0) is BLACK as 0 indicates BLACK color.