I just tried it on my system and I have no problem.
You are using a .icns file, right? If so and following the regular procedure (i.e., drag the new .icns file to the Get Info icon area of DEVONthink.app), I have no idea why it is not working for you. 
Dock icon works but not global inbox icon. Global inbox’s what I was referring to.
Are you trying it in the right place? The folder should be:
/Users/[your user name]/Library/Application Support/DEVONthink 3/Inbox/
And after that you can make an alias (and place it anywhere) or drag it to the dock (doing a shortcut).
The next release will support this.
Will iOS support coloured database icons also at some point?
The next release of DEVONthink makes the color of tags scriptable, this is not possible on iOS. Custom icons might be supported in the future on iOS too but no promises.
Just wanted to say THANK YOU! to @valente for your generosity in taking the time to share all of this (repeatedly!). Also thanks to @apoc527 for the tip and examples about iconjar, and to all other who’ve made helpful suggestions about this.
What a community this is! – would that other online communities were like it. (Sadly, they are not, and I’ve just about stopped frequenting any of them other than this one).
Thanks for sharing your experience and thoughts.
We are privileged to have a light, relaxed, and helpful community! 
For whatever it’s worth, I’ve been using FolderMaker to create custom icons for some time.
How to revert back to the original icon?
Select the icon in the Info inspector and press Delete/Backspace to remove it. Or choose Data > Thumbnails > Remove for multiple selections.
Awesome work. Now I’m trying to create a group with the exact color from Obsidian (#403A83 or something very similar) but I can’t figure out how.
Hi. I did a new group folder icon with #403A83 color.
Updated link for all files: http://u.pc.cd/ugS7 (see folder Group > Group 21.icns).
Is this what you need?
Yes! Thanks a lot!
I’ve done some new Global Inbox folder icons. 
You can find it in this link in the folder “New Global Inbox”. To set them up check the instructions also in the link.
I’m currently using the purple one, for a change of pace!
