What in the world ?? Rainbows ? are they looking at darn rainbows and butterflies ??
Basically a rainbow attack is an attack with one main army type and 50 - 1,000 troops of each other type you have available, usually not including siege weapons as they are too slow. You dont want to use less than 50, as they may be killed off in traps before they reach the enemy troops and then they will loose their purpose. The purpose of including them is to make the enemy troops busy for one round so that the main type of your army can either reach the enemies without being damaged, or fire at the enemy without taking damage themselves for several rounds.
This is called layering and here are two standard Evony rainbow formations.
In the first one you want enemy troops to fight off and shoot at these 1k groups before reaching your archers, so the archers have longer time to fire without being attacked. In the second rainbow you protect your cavalry, having the 1k groups fight and not make the cavalry stop before reaching the enemy archers.