It is fairly simple to make your own colours for models [
rename the file extension of the file data<date>.pk3 file into a .ZIP file , explore the .ZIP file, go to textures, find the model you want to edit's textures, open up the corresponding file, edit the .TGA with whatever and save over,
rename the file extension of the file back to a .pk3].
On the other hand, for models it requires the knowledge of modelling, preferably using the open-source modelling tool,
Blender 3D
Then it requires animation & exportation into a .ZYM file [the model file, including animations, that Nexuiz uses], and finally addition to ~/data<date>/models/player along with a relating .TXT biography plus path to the .ZYM file, and optionally a corresponding .JPG picture of the model.
Be aware that only you will be able to see the modifications/addition of your new player model, unless a renowned server [ie RBI CTF/Keyhunt] distributes an additional .pk3 containing only the model folder, and the player folder underneath, containing your model, .TXT, and .JPG.