These were young frogs about 15mm or so. I've never seen so many together. I didn't realise at the time but the dark one does seem to be very skinny, hope it's not that wasting sickness that's been going around.
You could try but there is not much they can do. My captive frogs can change their colour and to some extend size very quickly. A smaller frog could also simply just be a bit younger. The darker colour could indicate that it is cooler than the other frogs in the pics.