With the colors upgrade, you get:
All 6 theme colors, 3 backgrounds (light, dark, black) and 6 accent colors (configurable in settings design and in the settings for each widget)
Additionally in aCalendar 2:
100+ calendar colors (instead of 36) configurable in the calendar list
and extended event colors:
Always all 36 event colors (even when the used calendar offers less, e.g. Google Calendars offer 11 colors on Android)
NOTE: extended colors can only show in aCalendar, not in other calendar applications (but they do sync across devices).
See also event and calendar colors.