Rainbowfish is a common trade name that refers to over a hundred species grouped in 10 genus of which the Melatonaenia is the largest one. There's at least one fish that is commonly labeled as Rainbowfish that is actually a "false" one (common name Celebes Rainbowfish, the Marosatherina ladigesi).
There's a free downloadable book that I consider a very valuable resource of information for my yet young fishkeeping experience with Rainbowfish (since 2008) which you could download for free here:
http://members.optushome.com.au/chelmon/Book.htm
As juveniles they won't say much really, they look quite dull but as they mature (it can take over a year to do so) they can be stunning. The most colorful among the commonly available in the trade is the Boesemani Rainbowfish.
The most striking coloration of some Rainbowfish will take place only at mating time as does the M lacustris males display of intense yellow mark in its nape (forehead). It seems that environmental variables -diet, water parmeters- do play a significant role producing notable visual differences within fish specimens from the same species.
I keep a small school of (15) Glossolepis incisus, a large shoal of (90) Iriatherina werneri, a mid-sized shoal of (30) Melatonaenia boesemani and a small school (8) M splendida splendida. In the past I kept a mid sized school (30) of M lacustris.
A great fish for planted tanks.
Pepetj
Santo Domingo