We ... kind of need a lot more information about exactly what you were doing and *how* before we can figure out what exactly was happening here.

So let's start by covering some bases:

1 - What tool were you using and what were its settings? Nominally it should be "Normal" blending mode and some Dynamics preset other than "Dynamics Off" (as that would dismiss the entire problem as just user error)

2 - Did GIMP actually detect the tablet upon startup? (very well-known bug) The fastest way to check is to alternate between the pen and eraser tips -- GIMP should swap to a different tool depending on which end is in range. Otherwise, go to the Edit menu and check "Extended Input Devices", which should show the Wacom tablet in its list. If it doesn't, then GIMP failed to notice the tablet at all, and is only just reading it like the mouse pointer (meaning no pressure sensitivity or tilt control, though other Dynamics options may still work) and you should restart GIMP before continuing.

-- Stratadrake strata_ranger@hotmail.com
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Numbers may not lie, but neither do they tell the whole truth.

From: gimp-user-list  on behalf of Elise Knuuti 
Sent: Sunday, August 20, 2017 2:20:09 AM
To: gimp-user-list@gnome.org
Subject: [Gimp-user] Wacom and GIMP

Hi. I have a Wacom Intuos4 and GIMP. The colors don't blend. I try to take
a color and blend it with another but they just lie against eachother like
they can't blend.