The update will roll out starting today and will continue in waves over the coming weeks for those that are in the United States and have selected English as their language. It appears that the colors will be randomly associated with each voice, so everyone will have a different allocation of colors against voices in the Assistant preferences.
You can see from the GIF that the selection process is quite colorful and fun. Again, this update has just started rolling out today and it might not hit your device for up to a few weeks. If you want to check whether you have the new option, heading into Google Assistant, Settings, Preferences, and finally, Assistant voice. If you see a list, you haven"t been updated.
Source: The Verge