Freedon Nadd Short Saber?
 

How come his lightsaber is yellow when he was a Sith? It's supposed to be red isn't it?

jedi and sith are not defined by the colour of their lightsaber. Whilst most dark siders and sith do utilise the more traditional red crystal, not all did. Freedon Nadd, aswell as Exar Kun were both jedi originally and they both kept their respective coloured lightsabers, who knows as to why, perhaps it was no significance t them, or perhaps a constant reminder of when they were "weak" to give them strenght or perhaps that the construction of a lightsaber is more spiritual for a jedi than it is a sith, more in depth and ultimately more rewarding.

Also, the colour crystal except for red, were natural, the red lightsaber crystal was of synthetic design and caused the odd short out. Perhaps they kept their original sabers purely out of being more reliable.

hell, there were even some jedi wh remained jedi who had red lightsabers.. it is just stereotypical, but not mandatory, for the bad siders to use red.

Yeah like Xavier sed Freedon Nadd and Exar Kun kept their respective coloured lightsabers

Well i'm not really familiar with Freedon Nadd and Exar Kun as Sith Lords. I never read the comics or novels that had them in it. Only Star Wars novels I read was back in High school, the Timothy Zahn Heir to the Empire trilogy.

As I understand the reason that these first round sith kept with traditionally jedi colours is because they both had no time to change them and they didnt care.
The traditional red saber probably didnt come until the more united sith closer to revan's time

Unfortunately, I don't think there's a whole ton about Freedon Nadd's fighting tactics, but I do find it odd that he had a short saber and blaster. I bet it was awesome if he used them at the same time, I also wonder if maybe he had a regular sized saber for his other hand.