I don't think there is a standard.  Except that they must be unique in
their combination of shape, color, and markings.  When a pharmacist
checks pills that a technician has filled, he compares them to a picture
of what the correct drug looks like.  And identical drugs manufactured
by different companies must look different.  That prevents a generic
manufacturer from producing something that looks like a name brand,
which unscrupulous people can sell as the name brand.