i own a brava with a paint code 106(fiat dealer code) but when i try to buy there appears to be two different colours listed red siena and red noble both seem to have the same code

On the bottom edge of the bootlid should be a sticker with the color code and name. You should check that.

Probably one is a metallic and the other is a "normal" paint.

Welcome to the forum Have a look at this;
https://www.fiatforum.com/bodyshop/180981-bravo-brava-paint-codes.html

According to the original Bochures there was never a 106 on a Bravo or Brava, Worth mentioning my Bravo 1.4 had the wrong paint code sticker in the boot.

Maybe post a photo of your car.

paint code 106 belongs to the mica red from the new bravo (maranello red).
So or you posted this in the wrong forum and made a typo or somewhere, something went terribly wrong.