I don't even know they have different color lol

Nepgear's front hair(the middle part) point toward to our left. while Neptune to our right.

Also on the bottom of the neptune photo, you can see a little bit of the background, which mean she has short hair and not covering the whole background.

First- you might not notice but Nep Jr. has a diffirent hair colour (her hair are a little bit lightier).
Second- Hairstyle (cant explain in words)

↓ this Nep is right, but you need to gives me a explanation for your answer

Bottom: Nep Jr.
Top: Nep.
(im nepped in the head if this is the wrong answer) :D

I gives u guys a clue. U can determine which one is which by their hair

The more- the neppier! :D

the mold of all neps absolute

↓ that answer

Pretty easy