Lightsabers don't automatically turn red when one turns to the dark side. Anakin never bled his kyber crystal in the prequels, explaining why his saber stayed blue. Anakin's lightsaber needed to ...

making for a peculiar-looking double-sided red lightsaber with a bend in the middle. Asajj Ventress' lightsaber might be the most similar to Rey's if these pieces of concept art remain accurate.