I was creating faces with the given vertices and noticed that some parts of my mesh have lighter color than the rest.

[![enter image description here][1]][1]


[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/kjT6F.png
  [2]: https://i.sstatic.net/t2M7w.png

Can anyone help me with an explanation? Thank you so much!