Serrasalmus Sanchezi can be red or purple, and are often times referred as red/purple throat spilo
Serrasalmus Spilopluera is commonly referred to as Ruby Red Spilo, there are 2 varieties I know about, gold and ruby red. They are usually the most aggressive Piranha species and the reds are very rare, 8-10x more money then a Sanchezi and gold Spilopluera.
Both reach about 4-6"
If it were me id choose ruby red Spilopluera!
Go S. Vettel #1 rb8