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.