New to Turquoise Rainbows

I’ve never had rainbows before, I got 4 about 2 weeks ago. Today I noticed the largest of the 4 juveniles turned bright yellow on his head. Which led me to wondering the sex of the other 3. One of the others seems to have a little gold on it’s head today, but not close to the brightness of the larger one. I’ve got some photos let me know what you all think. I emailed them to myself so they should meet the size limit.

Melanotaenia lacustris (turquoise rainbowfish) change colour quite a lot and have distinct day and night colouration. In addition to their body colour, male rainbowfish produce a dorsal blaze when showing off. A dorsal blaze is simply a coloured strip running from the first dorsal fin down to the top of the mouth. In some species it is yellow, others are white and some are pink. The males are the only ones to produce the dorsal blaze and females do not produce one.

Rainbowfish need some plant matter in their diet and they will eat algae, aquatic plants, and sometimes fresh fruit and veges that have been washed and peeled. Duckweed is a small floating plant that does well in aquariums and is eaten by most rainbowfish. Using a goldfish flake or a vegetable flake/ pellet is also helpful for rainbowfish.

There is more information about rainbowfishes at the following link.
http://rainbowfish.angfaqld.org.au/Melano.htm

Thanks for the reply.