I am new here and Just started using Go SMS Pro. I would also like to change the app drawer icon but cannot find any info on it. But I also had the same problem of seeing the colored icons somewhere,
but what I found was it is not the app drawer icon it is the notifications at the top left corner of your screen that can be changed to a different "icon" or color.
Go to settings in the app, then Advanced, then Notification Settings, then Default notification settings, and you will see Notification icon.
I hope this helps.