Currently there is no option to customize category colors directly from the Android app.
But don't feel blue.
We're adding the ability to change the colors of categories in a future update! Until then you can change the colors of the categories on your account through www.outofmilk.com and have those changes reflect on your mobile account. 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.
Log out and log back in to the app on your Android device to see those changes reflected in your category :)