This is a wrong assessment. Alright, let me explain.
Kitana and Noob were both going to be on the main roster in MK4. However, the MK team decided to make new characters instead. So therefore they took Kitana's model and Noob's model and used those assets to create Tanya and Reiko respectfully.
A palette-swap is when taking an already established character model/spriteset and recolour it. Hence the name palette (the colours used) swap (changing those colours to other colours).
That is not the case with Tanya and Reiko though. Reiko got a new head instead and his gauntlets are unique, and Tanya's costume and hairstyle were altered as well, so they would NOT be palette-swaps. Jarek shares the same story, where initially Kano was to be in, but was changed into a new character.
In MKA for the Wii, Khameleon needed a 4th special move to fit Chameleon having 4 special moves from the male MK "ninjas", but there were only 3 female MK "ninjas", those being Kitana, Mileena and Jade. They could have given her a move from Frost (who is basically a female representation of a male MK ninja), but she was blue like Kitana, so that wouldn't work. So they gave her Tanya's fireball.
Tanya has never been a ninja though. Nothing about her is like a ninja. She wasn't even one of Shao Kahn's assassins like Kitana, Mileena, Jade, Reptile, Noob, Ermac and Rain were (Funny how all the palette-swaps with the exception of two originals, Sub-Zero and Scorpion, have all been Kahn's assassins *Edit: How could I forget about Smoke?! XD). She is dressed like a female Edenian, just like Sindel is, but Sindel isn't considered a ninja because of it.
But fine. If you want to call her a palette-swap ninja, then you call her that.