Thanks for the reply, Swaroop!
That definitely answers the main question of how to change the Automatic/default palette.
Unfortunately that solution also seems to change the normal behavior of the automatic palette, which alternates between sequential and diverging depending on whether the measure data has negative values (and is what I'd essentially like to happen, I guess); changing the colors in the Preferences file appears to essentially turn it into just a diverging color palette (where the Center is automatically set at the midpoint between max and min).
I may have to settle for just setting the Center at zero, but I'll keep playing with it. Thanks again!