To customize your category colors you must do so directly on from OutofMilk.com. Your customizations with then reflect in the app on your Android device.
We're hard at work adding this functionality to the Android app in a future update!
Until then, here's how:
1. Log in to your account and from your available category actions, select Manage All Categories.
2. From the My Categories tab, drag the category you would like to edit to the pencil field.
3. You'll see the Modify Category section on the right update to show the category you elected to edit. Adjust the color tools to select a new color. Once you've got your color, select Update Category to save your changes.
Your move, Hovannes Adamian.
Log out and log back in to the app on your Android device to see those changes reflected!