There are many different blade colours that the Sith used during the Old Republic era within SWTOR. Whilst Red was the most common, traditional and came in both natural and synthetic forms, many other Sith preferred the use of Orange, Purple or Yellow to make themselves stand out and represent the powers that they harnessed.

However, according to the lore of Sith Weapons, a Codex entry can be obtained by both Warriors and Inquisitors on their final trial before leaving Korriban that details the fact that Sith within the era have been known to use both Cyan and Magenta crystals. Two colours more typically found within the ranks of the Jedi.

While very rare, there have been some instances of Sith within the era that used these colours. Dread Master Calphayus wielded a Cyan double-bladed saber. the most famous example of any Sith to have used a Magenta crystal is likely Mara Jade who was the Emperor’s Hand of Palpatine during the Galactic Civil War.

You can obtain your own Legacy Bound Cyan or Magenta crystals from the Eternal Championship on Zakuul, alternatively some players can trade you crafted versions or can be bought from the GTN.

Sources:

Codex Entry: Sith Weapons – Warrior & Inquisitor only, Korriban
Dread Master Calphayus – Dread Palace Operation