You can make changes to your subscription to Pigment iOS, Assembly, or Marvel: Color Your Own from your iTunes account settings.
- Go to the Settings app from the home screen
- Tap “iTunes & App Store” > “Apple ID” > “View Apple ID”
- Login using your Apple ID
- Tap “Manage” under Subscriptions > “Pigment”
- Tap “Cancel Subscription” at the bottom. If you don’t see the button, that means you’re not subscribed.
Detailed instructions with screenshots here: http://pixiteapps.com/blog/basics-changing-cancelling-subscription/