Quotes from the Heresy: World Eaters

The World Eaters were First Founding Space Marines, and still regard themselves as such.

Even before the Horus Heresy, the World Eaters were noted for their bloody-handed approach to warfare and savagery of their training, Chapter rituals and treatment of enemies [...]

The World Eaters wear armour in Khorne's colours of red and black. Individual patterns vary, but the right gauntlet is always painted red as a mark of loyalty to Khorne [...]

However, the original chapter colours of white and blue are still visible on some items of World Eaters armour and equipment. Often a shoulder piece, a breastplate or a single piece of armour has come from one of the Legion's original Space Marines and has been incorporated (without redecoration) into the Legion's new armour [...]

Special Rules

World Eaters Legionnaires are subject to hatred of all mortal followers of Slaanesh, Space Marines, Inquisitors, psykers and spellcasters of any kind [...]

~Realm of Chaos: Slaves to Darkness, page 259.