White.
The color scheme comes from TV newscasts of Election night which go by the flag. On those TV newscasts red is one party (only in the Nineties did it become standard for red=GOP), and blue the other party (ditto the blue=democrat), and the undecided states have always been white (the third color on the flag).
Folks sometimes conjecture that "blue mixed with red yields purple, so an undecided state must be purple" but if you're going by Election Might news (which in turn go by the colors of the flag) then its white.