Basically you will be able to customise your Tardis more, make it stand out from others Tardises. You'll be able to do this by changing how it visually appears.
From the Tardis GUI you will be able to select the part of the Tardis you wish to paint and then from there you'll be able to select a color, and if it has an edit option for the selected piece you will be able to directly edit the texture.
Yes. It will be fully compatible with multiplayer. However a both the server and client will have an option to disable this and just return to the default look.
If all goes well it should be in Update 44!
I do think that the interior should get some form of customization, like in gta you can change certain apartments but it doesn't effect the actual apartment layout but the theme. Plan B is to be able to change the interior from inside the tardis to already existing ones.