I don't think you can edit the default palettes but you can add your own palettes in:
if you want to use the existing tableau palette and add you own you could use the free Colour Tool provided by InterWorks:
Where you can drag and drop an Image into the browser and it'll generate the code required for your preferences file in the My Tableau Repository folder on your machine. if you dragged an image of the default palette into the colour tool that'd get you started, then you could add the other colours you'd like in too.
I don't think that we will be able to edit the existing color palette in Tableau. Instead we will be able to create our own custom color palette by editing the preference file or using color tool for tableau
Swarnalatha Mannar Mannan