A few comments:

-rename "show coloring" to "update result"

-mask-editing (painting color marks) should be locked when color marks are not visible

-it's a bit weird that the lock-alpha button is enabled by default, and kinda disables the mask when un-checked.
would it make sense to not show it at all?

Really too slow:
-while it computes the result, krita doesn't respond/redraw anymore until it's ready. would be better if we had a notification or something like that.
-I see it uses only one thread, multi-thread is really not possible?

also: icons are coming ;)

sorry I got confused in my last comment, because icons order is not the same as in the tooltip...
so:
(-rename "show coloring" to "update result")
-rename "update" to "update result"

(-it's a bit weird that the lock-alpha button is enabled by default, and kinda disables the mask when un-checked.
would it make sense to not show it at all?)
-That is the show coloring icon, of course.. (I got confused as with marks hidden too, nothing is visible..)

There is some fancy code in here, so my feedback only relates to using it.

I tested out the branch and it seems to work pretty well as-is. It took me a bit of time to realize you can use the tool without even opening the tool options. You can do everything with only the layer properties. Pretty nice. A couple things that might make it work a little better.

1. If you have the lazy brush tool selected and start painting on the canvas, create a colorize mask layer and start using that if it doesn't exist.
2. The processing seems to take a bit of time. There probably needs to be a progress indicator/busy icon that Krita is working on it.

Looks great though. Good job Dmitry!

Just tested the krita-lazybrush-kazakov branch:

1. Key strokes are only visible in 8-bit. When switching to 16-bit or above, I can see the transparent preview of the result but not the stroke itself. http://i.imgur.com/jU1Fxrx.jpg
2. As a user it took me a while to figure out that my usual smudge brush presets etc. weren't working. Shouldn't we automatically switch to a more appropriate preset when the colorize mask is selected?
3. I want to use the colorize mask to produce alpha locked paint layers for each colour, what's the most convenient way to do that? Currently all I can find is converting to a paint layer and then using the split layer function, but that destroys the mask in the process.
4. I tried to work around that but I'm getting rather consistent crashes when duplicating the mask or the parent layer.

I am getting a segfault whenever I try to create a new document. I don't even get to the new window. As soon as I select File > New, this comes up in gdb

Thread 1 "krita" received signal SIGSEGV, Segmentation fault.
0x00007ffff242dec3 in KoColor::colorSpace (this=this@entry=0x81730bc)

at /home/scott/kf5/src/krita/libs/pigment/KoColor.cpp:261

261 return d->colorSpace;

Going File > Open works, so there is something specific that the new dialog is requesting that is messing it up.

After working around that crash by opening an image I continued to test. What you have is quite nice. The convert to paint layer seems to be working well. It is really nice that it isn't tied to the linework layer any longer.

@sketchstick - Good points about 1 & 4. I am also getting crashes with duplicating. This feature in general was too big of a scope for a stretch goal, so points 3 & 4 probably won't get done unless they are easy for Dmitry. Doing this work is already weeks over budget. We are kind of concentrating right now on getting a specific workflow going. We really need some good documentation for this. Not following the happy path with this can easily lead to confusion.

The patch has been merged in 94dd517589492850f2c31c3aea8fef2d111ecf27

Please report bugs in T372