MyPaint has an "instant"/hovering color picker on the stylus button, whereas GIMP gives you the main toolbar menu again, and there's no setting to change it yet, unlike in Krita.
While holding control and then clicking/touching the stylus tip is not really a cumbersome thing to do, if one is used to MyPaint's way, which is slightly more practical, there's a hackish way to do it:
#!/bin/bash
xsetwlock=0
while true ; do
if xdotool getactivewindow getwindowname | grep GIMP$ > /dev/null ; then
if ((xsetwlock==0)) ; then
xsetwacom set "Wacom Bamboo Pen stylus" Button 3 "key Control button 1"
xsetwlock=1
fi
else
if ((xsetwlock==1)) ; then
xsetwacom set "Wacom Bamboo Pen stylus" Button 3 "button 3"
xsetwlock=0
fi
fi
sleep 0.3
done
That's for my Wacom tablet model specifically, one of the simplest bamboo ones, it needs to be adapted with the proper device names for different models, and I have no idea about non-Wacom tablets.
You just leave the script running as you run GIMP, and it will loop checking whether the focused window has "GIMP" in its title, and if it is, it changes the third button to "control click", which "automates" GIMP's way of activating the color picker, in a way that mimics MyPaint's somewhat. When GIMP is not focused, it restores the button to its default value.
It's not perfect, holding the button and hovering around won't change the color picked on first button click, but it does not require holding control and touching the tip, so it's a much closer dynamic.
Another potential issue is that, while it's running, it will change the stylus button in any window that may have its title ending with "GIMP" for some reason.
It has no output whatsoever on the screen. If you remove ">/dev/null" it will "echo" the active window title containing "GIMP".
Maybe the sleep time can be considerably extended without causing any trouble, to make it lighter, CPU-wise, even though I guess it's not really heavy.
Requires xdotool and xsetwacom.