this works with ANY colors you want, but i took these :)

 first you need blue nailpolish, as the "fundation"

 then i took a ligh purple nailpolish and just stroke a line in the middle of the nail.

 ´for this part you will need an art pencil, or a toothpick, but its much easier with an art pencil.

 then you make small lines on the (in this case) purple line. 

then you turn the lines into small crosses

 the last touches are the small lines on the sides of the purple line.

 DONE! :)