Philae, the probe that landed on a comet, used Forth.

https://en.wikipedia.org/wiki/RTX2010

ChipWits was a programming game written in MacFORTH.

https://en.wikipedia.org/wiki/ChipWits

Chuck Moore, who invented FORTH, uses his own CAD system written in his own Forth-like language to design Forth chips. It is a very complex task and Forth discipline is strongly enforced in the whole process. CM also wrote colorForth, which includes what you could call a simple panting program (it is used to draw font glyphs).

In embedded environments like control systems or CNC, it is also quite common to find some kind of Forth. These are not exactly painting programs, but they are used to paint with laser or water jet on real world materials, which is cooler.

And, although it is not Forth, postscript follows similar principles. You could consider ps as a quite advanced paint program.

There is the "Workstation in a Mouse"

http://www.ultratechnology.com/scope.htm

Bit of a toy demo though since it only ran a single fake program.

A chess game: http://www.ultratechnology.com/chess.html

The largest serious suite I know of is a VLSI CAD/simulator for silicon chip design:

http://www.ultratechnology.com/okad.htm

http://www.ultratechnology.com/okad2.htm

Pictures of it in action: http://www.ultratechnology.com/tape1-2.htm

I almost said "uh, Starcraft was written in Forth" but I was thinking of Starflight. (https://en.m.wikipedia.org/wiki/Starflight)

forth was used in the philae lander, http://www.cpushack.com/2014/11/12/here-comes-philae-powered...

Look here for a colorforth like system: https://github.com/phreda4/reda4

Many programs, 2d, 3d, editors, voxels, compilers, etc ..

Everything is written in that colorforth inspired language.