Some new pictures – I first thought that the differences in colouration were an artefact. But it isn’t – it’s maybe aggressive colouration, they had been chasing each other. The one that stayed yellowish is the bigger one I believe to be a male (see above). Faintly, but still – you can see the pattern in it’s tail fin on these new photos, too. The silvery ones don’t show the slightest tail pattern at all – angry females?
After a while – all show the same yellowish colouration again (supposed male in the middle):