I do not think you can remove default colour palettes.
You can use the Preferences file to add a new colour palette - assign all the colours you want users to use - based on your company style guides etc.
and instruct the developers to always use the company palette.
We usually add 2-3 different palette - the first one including the various colour and 1-2 further palettes for continuous colours for measure values / RAG colour schemes etc.
u can go to tableau repository and open preferences.tps file and paste the following code like:
<color-palette name="Classic 19" type = "regular">
remove the previous workbook tags from there and paste these lines.In the colour tag paste the code of the colour which u want to be shown.U can search for rgb code for any colour from google.
If u want to create a pallete of more than one colours then add one or more colouur tags beside previous one(like <color>##ffffff</color>).then go to your sheet canvas and click on colours then select edit colour and click on assign pallete and open the dropdown of select colour pallete where u will find your colour.