Feathered Fiend

Feathered Fiends are twisted mutations spawned by Tzeentch’s influence. Feathered fiends are birdlike creatures created when a human eats food tainted with warpstone dust and ritualistically infected by the Cult of the Broken Wheel.

If that human is a wizard, the effects are more severe and varied, resulting in a Coloured Fiend – a feathered fiend with a connection to one of the colours of magic. Each colour fiend looks and behaves differently. They are parodies of college wizards and appear at first glance similar to a wizard of that order, only strangely exaggerated.
 * Amber Coloured Fiends -- The amber fiend appears as a huge, hairy man-like beast, seemingly dressed in ragged furs. Its eyes are wild, its teeth like fangs and its fingers long claws. It attacks by pouncing upon its victims like a wild animal and tearing at its flesh in a bloody frenzy. An amber fiend can use Ghur spells.
 * Blue Coloured Fiends -- The blue fiend appears as a lanky figure in long, flowing robes, covered in shimmering stars. Its body insubstantial, no flesh can be discerned. The stars float around the figure as if independent of it. A blue fiend can use Azyr spells.
 * Green Coloured Fiends -- The green fiend looks like a large muscular human with ragged hair bedecked in a cloak of leaves and thorns. Where it steps the city becomes suddenly lost to nature. Cobbles and bricks crumble to sand. Weeds and shrubs grow up suddenly. A green fiend can use Ghyran spells.
 * Grey Coloured Fiends -- The grey fiend is almost a living shadow, barely discernible, except for glowing red eyes. It leaves long tendrils of darkness in its wake, like a cloak. In combat, it prefers to emerge from the darkness to strangle its victims with illusory hands. A grey fiend can use Ulgu spells.
 * Purple Coloured Fiends -- The purple fiend is dressed seemingly in the darkness itself, trailing an air of emptiness and even death in its wake. It floats above the ground and its empty black gaze brings terror to any who looks upon it. A purple fiend can use Shyish spells.
 * Red Coloured Fiends -- The red fiend simply strolls through the city in a billow of angry smoke, setting fires wherever it steps, leaving scorched ground and embers in its wake. It takes the form of a blazing pyre in human shape, its two eyes burning even more brightly than the rest of it. A red fiend can use Aqshy spells.
 * White Coloured Fiends -- The white fiend appears as a ghostly, shining apparition, bedecked in silver and white. It floats serenely through the air just above the ground. Everywhere it casts its gaze burns under the brightest light and when it is attacked it shimmers and becomes suddenly ethereal. A white fiend can use Hysh spells.
 * Yellow Coloured Fiend -- The yellow fiend appears as a shining statue of pure gold, brought to life. It wields a sword and shield and is bedecked in golden armour, with a long golden cloak trailing behind it. A yellow fiend can use Chamon spells.

Source
pg. 30
 * Warhammer Fantasy RPG 3rd ED -- Book of Change