There are some colours you can't change and they are indicated with square brackets like "[Registration]". In Illustrator you cannot edit the [Paper] colour represented by 0,0,0,0 also [Black] is one of those colours that can't be changed. If you want an active white you must create that colour as a Spot colour. A spot colour it is the name that is important as it will be swapped out in the RIP. If you work in CMYK mode you can view spot colours and separations seperately. Sometimes it is more practical to make a white base as the top layer and check Overprint in the Attributes, and at the time of printing you choose which order to lay down your ink. (this is often simpler than putting white base below and makeing all other objects overprint since overprint may cause unwanted blending of other objects).
Here is also a link to one vendor suggests a method for creating a spot white.
https://www.youtube.com/watch?v=9W6JprByCOM
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