I’ve been told that when checking rice for insects that all grains that a have a black spot or discoloration must be sorted out. Is this correct, and are there mareh mekomot that could be sited? Thank you.

26 Iyyar 5771, 41st day of the Omer The most common sign of the rice being seriously infested are webs clinging to the rice grains and a powder like substance at the bottom of the bag. Black spots or burn like marks on rice are not a sign of infestation but are a mere defect in the growth of the rice. However, bugs in the rice may be brown or black and also white. (For details see Rabbi Moshe Vaya's book in the chapter of dealing with the inspection of legumes)