Is there a way to set the shortcut for the Color Selector Tool to the the middle mouse button/wacom pen button?

Or, setting a pen button in Ubuntu Tablet Settings to the "P" key to do the same thing?

We are using Wacom Intuos 5 touch tablets on Ubuntu 16.04 and Krita 2.9

Thanks!

I'm not sure what is the equivalent in Ubuntu, in Arch/Plasma5 I use the wacom-utility, this is just a KDE front end of the xsetwacom (thus may be others for different DE), it appears (since I haven't try, but the options are there), that you can in fact map a key to the stylus buttons, you must understand though, that once done it will work the same globally, which in turn, my be solved by using a "profile" that you will only use while painting on Krita and then switch it on demand.

Again, I'm not sure if that would work but I think is worth to give it a try, thus I recommend to look for xsetwacom information (as can also be done manually) and its front ends (gui) to see what's possible.