Press MENU, then SETUP
Scroll to MAP COLOR (DAY) and/or MAP COLOR (NIGHT)
You can select from 5 day and night factory preset colors (NORMAL, #1, #2, #3, #4).
These instructions are on page 93 of my NAV manual.
You can also adjust the brightness and contrast of the display to suit your preference. Instructions are on page 115 of the manual.