hmm, I'm not 100% sure with xrays but I don't think their colours are going to indicate any difference. With some species of tetras the males are more intensely coloured, and there can be some difference in fin-size. However the females will have obviously larger bellies (some pet store only sell one gender of fish, so don't be too surprised if you don't have females & males; but it sounds like you have 2 girls and some boys!).
Now tetras scatter their eggs so if you want babies you have two options (I don't remember what else you had in this tank). You can separate the females into another tank (carefully! you don't want to change their environment a whole lot or you are going to stress them, I can tell you how to do it). Or for a moderate result (which you may want) you can lay a net across the bottom of the tank & keep it well planted. The second option is going to result in few babies surviving, but that isn't necessarily a bad thing! http://animal-world.com/encyclo/fres.../pristella.php
Here is a great page on the little characins!
I still recommend a large tank with a carnivorous cichlid for the krebs
. You are well on your way to becoming a bone-fied fish-head! Be careful, it's a slippery slope! Multiple Tank Syndrome (or MTS) is a widely diagnosed disease.
A general note:When purchasing fish such as xray tetras, be sure you are purchasing naturally coloured fish, not those who have been dyed neon colours. It is a fatal and cruel practise.