I can't say for sure about the mileage, because I haven't done that calibration. But on the color, I finally just discovered something the other night.
As background, I've always found the SG to be a bit bright at night, but I can't use the "half bright" setting because it's always "flickering" (I'm one of those people who can see monitors, etc. flickering - friends of mine can't see it, the lucky dogs). The full brightness doesn't flicker, but was always SO bright at night (or, if I chose a dark color, not enough contrast). Picky, aren't I?
I had seen the "custom color" option, but just figured it would be something complicated and so never really checked into it. Well, the other night I did. It couldn't be simpler! Basically it just takes you to a screen with three colors listed on it, each one has a button matching up to it. You can cycle the button to chose 0, 1, 2, or 3 "parts" of any or all of the colors. Mix and match. The standard blue-green always looked nice on my dash, but now I have a sort of a mustard color that's visible during the day, and much less bright at night (even on the bright setting).
I can't believe I drove with it the other way for almost four years... geez.