Why are there shades of blue eyes that are lighter and others more darker?

  The answer lies in the density in the iris, very light blue eyes tend to have the iris denser than darker blue eyes, so not all people who have blue eyes are perceptible at first instinct, because the iris density is smaller, all irises are translucent but light eyes with denser irises are lighter than normal, making these eyes appear much brighter in any environment and illumination.

 VERY LIGHT

Meg Foster has one of the lightest and rarer eyes of history, similar to the eyes of the Siberian husky.

Perhaps Meg Foster is the only person in the world with such light eyes that there are, although there are many celebrities like Ellen Degeneres, Dan Stevens, Daniel Craig, Ian Somerhalder,  with very light eyes, but  none have the same shade as his eyes,  (Maybe some Swedish, Dutch, German, Russian or other Nordic person somewhere in Europe may have but more not yet discovered)

Meg foster young

NORMAL BLUE

Taylor Swift also has blue eyes, however his iris is thinner, although in some more direct lighting the color blue becomes more evident.

Beauty princess

   The same happens with green eyes that are dark and others that are much light, but in some cases this factor is by the amount of melanin present in the iris.
  That's why we see so many shades of light eyes, some very light others darker.