Hazel eyes can be more greenish, more brownish and even blueish...... they almost reflect the colors around them.
I would say his eyes were hazel blue but as you point out they definitely appear different in various situations and lighting.
My eyes are bright blue, they never look another color no matter the light reflection or what I am wearing- Elvis's do look different in picture to picture depending on lighting and colors around him.
My wife has one brown eye-and one eye that is multicolored, so I would call that eye Hazel because it does at times appear like blue, green, dark gray, etc.....
I just browsed our gallery here 50s, 60s 70s, and it is amazing how differing his eyes look in color- sometimes even dark brown other times dark blue, light blue, steely blue, greenish etc........another fascinating thing about him.