open Creo dialog, where you can mix Creo system colors (geometry, curves, background, and so on) and try to find out which system color is associated with your symbol.
According to this, the System Color is Intrinsic:
map_color intrinsic_color 7
but that didn't work.
I did click on Color and change it to "Letter" and when it was exported, it became white
Now how do I edit the symbol in the library?
Intrinsic color tells you, that the symbol contains several entities and these single entities have different colors assigned.
To edit symbol definition:
Intrinsic is just a button/indicator for the original color(s) the symbol was created with.
It should be 'default' but there's not enough GUI style QC.