The above video comparing the two apps' watercolor simulations and illustrate several differences. First, we use physics to model the deformation of our virtual brush, allowing subtle variations in the strokes. Because it's a 3D virtual brush that you can wield, you can e.g. making a gradient by loading different colors along the brush tuft and tilting your brush just like in real life. Fresco uses stamp to model the watercolor brush, so the above natural brush manipulation is simply not possible. Second, we use Computational Fluid Dynamics  to model wet watercolor for really natural flow patterns. Close examination of Fresco suggests that they use expanding 'waves' to model spreading   watercolor. Such modeling often gives  artificial-looking 'turtle-back' patterns. Another important point is that our flow is global instead of being restricted to stroke vicinity like in Fresco. This keeps Fresco from being able to create large-area flow patterns - a major contribution to the charm of the watercolor medium. Lastly, you can't see subtle details in Fresco's rendering when you zoom in. In contrast, Expresii can render the artwork in much higher resolution with details. 

From the video, it seems the brush is only modeled as a circle i.e. strokes are made by sweeping a circular shape, scaled by the pressure applied. Years ago, there's a team from Taiwan making a tool named 'Royal e-Pen (神來e筆)' specific for Chinese calligraphy (video right) and that's basically the same software tech they have - modeling the brush as circle. When we were young,  we used to learn calligraphy by writing on top of model calligraphy (‘描紅’ as they say in China).  Teacher would say the worst crime you can commit is filling up strokes by going back and forth like what you do with coloring books. 

The other appearance of Eastern calligraphy in Apple's video uses a more elaborated rough brush footprint to give a bristle or scratchiness effect (below).  The app used seems to be Procreate, which is a very nice painting app, btw.

Two years later Jos Stam published a paper called Stable Fluids  [SIGGRAPH 1999].  He proposed to use a 'Semi-Lagrangian' method to solve the fluid equations so that large time steps can be taken in the simulation iterations to allow user-interactivity.  Because the implementation is simple and that he also released sample program for it, his method was widely spread. "Everybody can have his/her  own  happy meal and fluid sim",  so to speak.  At that time, usually simulation was done at a resolution of 256 x 256 pixels or lower.  

Later in 2003,  Mark Harris et al. adapted Stam's method to the GPU  for better efficiency.  Today,  maybe 95% of interactive fluid sim's out there (e.g.  this   &   this)   are based on Stam's work.  
​
Note that Stam in his  2003 paper  wrote:​

Above: A demo of Stam's 1999  fluid  solver.
​Below:  One of the many implementations by others.

acknowledging  that  what makes his solver special was actually something that had been done before.  While I'm not a lawyer, we are not sure if anyone implementing Stam's method for a profit would infringe their patent or not.  That might be one reason why many Stam's-based fluid sims we see today are mostly  'experiment' or 'toy programs' instead of commercial ones.

In 2003, our Nelson Chu started  working on simulating Eastern ink on paper.  He found Stam's method not very  accurate if large steps were taken to maintain 

Nelson's 2005 MoXi demo

Another painting simulator featuring CFD is Peter Blaškovič's   Fluid Painter,  apparently  started in 2009  (judging from the earliest comment there).  Peter didn't state what method he used, but it looks very much like one based on Stam's without much change.  His other paint program Rebelle  that came out in 2015 doesn't seem to model watercolor faithfully  since the spread is like waves that bounce back and forth off the boundary (see above) and the results lack minute flow details. They might actually be using shallow water equations to model the flow.

The   Verve Painter by Taron came out in 2014.  Taron  also didn't state what fluid sim method he used, but if he based it on Stam's, at least he adapted the fluid density to volume and added shading, brush options, etc,  making impressive results possible. 

In around 2013, nvidia  entered the tablet market with their own mobile chip & stylus. With that, they also developed the paint app Dabbler (2014-2015). They modeled the viscosity of oil and the flow of watercolor but it's not clear if actual CFD was used for watercolor since the effect is mostly just blurring away the colors, which can well be done with burring instead of CFD.
 
Oh, there's one more:​ Adobe's new Adobe Sketch for iPad that came out in 2015.   Adobe licensed MoXi in 2006, but apparently they finally didn't really make use of our fluid code.  However, now you see this watercolor sim in this 2015 iPad app, whose effect is similar to what we had back in our 2005 MoXi, maybe somehow simplified since the flow is mostly an expanding motion and seems no free flow (see the Expresii video on the right) is possible. Adobe usually publishes paper on their research work (like in their previous attempt on watercolor, which uses some improvised physics and polygons instead of CFD), but so far we haven't found any documentation describing the tech behind this.  

Okay, that's all paint apps with CFD for watercolor-like effect that we are aware of. 

p.s. There were also attempts to use CFD for thick, oil-like paint simulation (e.g. Bill Baxter's), but that's another long story so we restrict ourselves to watercolor-like painting here.

Update 2016/10/22:  I think we should also show our Expresii here too, just for the record.  The video on the right shows Expresii running on a mobile device, namely a Windows tablet powered by Intel processors.

Above:  Adobe Sketch's watercolor (2105)
Below: Expresii on a tablet (2016)