Determining the sex of this individual

[zenith8]

Thanks everyone, fingers crossed they'll make a pair.

[foluxe]

Your first picture is a female, and the second one's a male. Females have tails that are rounded, while males have that distinctive shovel shape. The dorsal fin on a female is slightly rounded at the tip and the very end is transparent, whereas on a male it's pointy and the color goes all the way to the end. Females also tend to have more yellow on their bodies.

Your female just doesn't have her colors yet. I've got a female that was almost colorless when I first put her in the tank, but now she's looking great. Here's a pic of her - she's the one at the top. And the male's at the bottom.