If you are set up with a regular GMail account you can change the calendar color from the website, and it will change on the iPad. Not always immediately, but eventually. You can speed it up by making sure you close the Calendar app in the multitask bar, then open it again. If that does not work try turning the calendar off, then on again on the iPad in the account's settings. One of those two worked when I tested.
I normally have my Google account set up as an exchange account, so that I can have multiple calendars from the same account. Changing the color on the Google Calendar site does not seem to affect how they appear on the iPad. I tried several things to force it but had no luck picking my own colors.
I did manage to change the colors though. I went into Settings >> Mail, Contacts, Calendars, chose my account and turned off the calendars. Then I accepted that all my Calendar entries for that account would be deleted from the iPad. Then I turned the calendars back on.
When I went back to the Calendar app it reloaded my Google Calendars. They both came up gray, but when I opened the calendar list they had new colors assigned to them, not the same colors I set up on the web site unfortunately. Hiding and unhiding them changed the colors of the events from gray to these colors.
I did this a couple times, and the colors were not always the same. So the iPad is picking the new colors semi-randomly. If you are patient you might get colors you like this way.
Best I can do for you.