To cancel your subscription, please follow these steps:

On your Android phone or tablet, open the Google Play Store.
Check if you’re signed in to the correct Google Account.
Tap Menu -> Subscriptions.
Select Recolor - Coloring Book subscription.
Tap Cancel subscription.
Follow the instructions.

Please note that uninstalling the app will not cancel your subscription.

Here is a guide provided by Google Play: https://support.google.com/googleplay/answer/7018481