Hi and welcome to the forum
male Melanotaenia boesemani have blue/ purple on the front half of their body, females don't. So if they show any blue on their face, they are males.
Females are primarily silver with a few pale horizontal lines but look completely different to males when mature.
Basically, blue & yellow are males, silver are females.
The fish in all 4 of your pictures are males.
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If you are interested in rainbowfish, there is more info about them at the following link. Just click the names on the right side of the page and it will tell you all about the different species
http://rainbowfish.angfaqld.org.au/Melano.htm