Good or bad rice ?

How can you know if the white rice that you buy have a good quality ?

So why do I ask this question, this is because my white rice that I buy sometimes I can find black rice, sometimes the rice is not totally black but it has some parts black or even can sometimes be yellow. Even if 80% of the time the white rice is white but I have found in one hour 54 seeds of black rice. This is not a lot compared to the ones haven't got time to take out from the sachet of rice. Normally rice is white but they are not always white.