The color of the calendar isn't actually stored in the .ics file, so far as I can see. The iPhone just assigns the colors in order of addition of the calendar and restarts the cycle if you have more than 5 or 6 calendars. If you want your calendar for OmniFocus reminders to be a different color, first go remove it by turning off the feature and deleting the calendar from your settings in the Mail, Contacts, and Calendars settings page. Then go to the
http://www.apple.com/downloads/macos...index_top.html
page and subscribe to a calendar or two. Now turn the calendar export in OmniFocus back on, sync, and go to the calendar. If you are happy with the color your reminder calendar was assigned, you can go and delete those other calendars you subscribed to. Otherwise, delete the OmniFocus calendar again, subscribe to another calendar, and try again.
The rant about Apple's "restrictions" is pointless, IMO. There is no restriction here, there's just a missing feature (choosing the color that a given calendar uses in the iPhone calendar app) that no one has implemented yet.