star wars - Why do the Sith use red-colored lightsabers, but the Jedi don't?

As far as I have seen, the Jedi have different color lightsabers viz. blue, green...but, all the Sith have red-colored lightsabers. I don't have a heavy grip on Legends, so I can't declare it an universal rule, but look at this:
In the last episode of Season 4 of Star Wars: The Clone Wars TV series, Asajj Ventress gave her one of lightsabers to Obi-Wan Kenobi and said, "I want that back."
Then, Obi-Wan Kenobi replied, "Alright. That's not my color."

It means that Jedi don't use red-colored lightsabers. My question: Why? I don't think color has something to do with Force use or abuse. Anyone can pick their favorite colored crystal to create their lightsaber. What's the problem with red?

Anakin was using a blue-colored lightsaber that he lost in the battle with Obi-Wan. But, after becoming Vader, he started to use red-colored lightsaber. Why do the Sith use red-colored lightsabers?

Answer

They're actually red because the Sith use them.

This is addressed in the novel Ahsoka.

First the book explains that the Force can change the colors of the crystals used in the lightsabers, mentioning that the dark crystals (used by the Sith) end up "bleeding" until they are "the deepest red". The crystals used by the Sith are naturally colored crystals typically stolen from light-side users, which are then corrupted.

Dark crystals were made, too, but not in that holy place. They were plundered from their rightful bearers and corrupted by the hands that stole them. Even rock could be changed by the power of the Force, bleeding alterations until their color was the deepest red. The balance was finely staged between the two, light and dark, and it took very little to upset it.

Ahsoka later explains this further when she shows off her new white sabers. She explains that she has restored them back to their original white after having been corrupted into their red coloration by the Dark Side.

“I’ve never seen white ones before,” Bail mused.

“They used to be red,” Ahsoka said. “When the creature had them, they were red. But I heard them before I ever saw him on Raada, and knew that they were meant for me.”

“You changed their nature?” he asked.

“I restored them,” Ahsoka replied. “I freed them. The red crystals were corrupted by the dark side when those who wielded them bent them to their will. They call it making the crystal bleed. That’s why the blade is red.”