Primordial

Progenitors 「原初の悪魔, Gensho no Akuma」 also known as Primordials or Progenitor Demons, are the oldest and extraordinarily powerful demons. According to Leon Cromwell, each of them is referred to by their signature colors, in place of an actual name.

However, each one appears to have a name, or something similar:

Purple Progenitor (Violet) Ultima

White Progenitor (Blanc) Testarossa

Yellow Progenitor (Jaune) Carrera

Black Progenitor (Noir) Diablo

Green Progenitor (Veit) Mizari

Blue Progenitor (Bleu) Rhein (Hirari in WN)

Red Progenitor (Rouge) Guy Crimson

Appearance
Black : Wears a full butler attire with white gloves. He has black hair with a strand of red in the front

White Has silver hair, snow white skin, and red eyes. She is very beautiful.

Red Has crimson red hair and is very beautiful.

Personality
According to Leon Cromwell, they are heretical and often troublesome because they fight using Technique and wits instead of Raw and Sheer Power like usual Demon.

Chronology
The term first appeared in Black and a Mask, although the term Primordial was used in the side novel story The Lake Shore Dyed Scarlet Incident.

Black was summoned by a woman one of two member of Silver Wings, who begged him to do something to 'them'

"If you grant my wish, I don't care if you're a demon! You can have my soul! So, please... Make them... Make them..."

Black agreed to her wish, and in exchange, devoured her emotions.

Black then hunted down an Archdemon, supposedly fulfilling his contract.

Abilities
at first the strength of Progenitor were similiar with Archdemon since they no longer could Evolve and Increase their Magic Amount which led them to Develop their own respective Technique and Skill which led the difference power between Normal, Named Archdemon and Progenitor in term of fighting skill despite having same amount of Magic and being Cathegorized as Archdemon.

Black's Abilities:

 Fire: The ability to summon fire

Claws: Black can produce very long, sharp claws on either hand

Speed:  Black is extremely fast, even to the point where it seems he is teleporting

Trivia
the Nickname of each Progenitor were from the french language of their respective Color.

There's no such thing as Progenitor in Web Novel due to Diablo's Hair color is Red and Testarossa's Hair color is Scarlet which completely contradicting with the Colored Demon in Light Novel. it's presumebly though that the Demon in Web Novel were only Red like Demonic being in Mythology.