It has recently been revealed that the act of a sith building a lightsaber will turn the crystal red. A Jedi turning Sith will not in itself turn the blade red. This is why the Skywalker saber is still blue(see answer here)
Another Jedi turned sith is count Dooku. Yet he has a red saber. Is it customary for sith to turn their blades red just because it looks evil? Getting a crystal and making a lightsaber are not simple jobs. Is there any canon answer to this?
The Jedi Order is shown to accept the lightsabers of those members that leave.
Ahsoka Tano left the Jedi Order at the end of the season 5 of the Clone Wars and gave up her lightsabers - Anakin returned these to her before The Battle of Mandalore.
Dooku had a bust in the hall commemorating those lost 20 Jedi that have left, so he left on good terms meaning he would have followed any rules like this.
When he became a Sith and needed a lightsaber it was easier to build a new one than to ask for / take his old one back, this would obviously have had a red crystal as per Sith lightsabers.
There is also evidence of this happening in Legends with the New Jedi Order
In the New Jedi Order when a Jedi leaves they surrender their lightsaber:
Tenel Ka did this when she left the order to focus on being queen of her homeworld.
"She held out her lightsaber to Luke. “It is with great regret that I must surrender this. I simply cannot fulfill the duties of a Knight in the Jedi order.
Although Luke gave it back to her
I understand.” Luke accepted Tenel Ka’s lightsaber, then pushed it back into her hand. “But please keep your lightsaber. You earned the right to carry it, and that can never be taken away.”
New canon pretty much messed up things with lightsaber crystals. Now we have this : Both Jedi and Sith are using Kyber crystals . Artificial synthetic crystals (usually red, usually used by Sith or other Dark side users) are now part of legends.
In new canon, while constructing his lightsaber, Force user will "attune" crystal to his personal energy "signature". Therefore, Jedi crystal would usually be green or blue, sometimes yellow or violet. Sith would twist and subdue crystal with their will, crystal would "bleed" and become red . Crystal would not change color if later Force user switches sides of the Force. Primary example would be Vader/Anakin, his lightsaber remained blue even when he killed younglings with it .
What we know about Dooku is that while still in Jedi Oreder he become a master of Makashi style, and constructed his lightsaber accordingly, with characteristic curved hilt. Of course, at that time his kyber crystal would not be red. What happened to his original lightsaber(s) is unknown, maybe he did turned them over when he left Order, maybe he kept it and switched crystal. In any case, his lightsaber could get red blade only after he was no longer a Jedi, because red blade and red crystal in new canon signify Dark Side .
