Well, you techinically can't. The rainbow diamond is the temp model that is displayed when you don't have the model on your computer.
For example, if someone is playing as the player model 'Suiseiseki' on a server and you don't have that model you will see that diamond model instead. Same story goes for any other models.
I've been thinking about that too (only recently though). Maybe I'll fix something else if it would be appreciated, both by community and the developers.