my sister did undyne but I colored her body.
I did samara (an asari from mass effect) where I used the same colors.
Different blue Aqua Makeup (Kryolan and Eulenspiegel) and a soft sponge work really well for it.
for the fish scales we used fishnet stockings and a darker shade of blue.
the details were added with a white eyepencil and red lipstick ^^
I hope this helps.