Ultimately any analogy will fail, because it's just that, an analogy - while having a pattern in common things are still different. You of course may disagree, because I also somehow already expected that, in the hope for a lively conversation and discussion about this topic. Yet I try to stress these parts, where such an analogy would apply, and disregard those, where it won't. The painting artist may "sample" a piece from the sunny room where he's in, or have a look out the window and just paint what he is seeing there. Musicians are very limited in their possibilities to imitate nature. But both, composers and painters, could perform their art from materializing the things that cross their minds or how they feel it should be done. Also van Gogh followed common painting rules or formed up his own for a certain piece, so he wasn't completely free in his mind when painting, just like a composer who has to think of instrument ranges or possible playing techniques. With today's synthesis methods we could "build" a violin going down to A-2 - is that desirable? Isn't it more interesting to see, how a composer would solve the "problem" with the tools he got at hand, a score sheet, pen and paper (or virtually said: midi channels)? That's a different view of what van Gogh was doing with easel and paints, and the flower in the painting being virtual as well.
The way I see it, the difference is, that the tools for a simple or complex painting always have been affordable while the tools of composing were unaffordable for most, even talented and/or successful composers. Sample libraries do change that, guys like you and me are able to lay down fractions of their ideas. But we are limited - we can do what the tools allow us to do. If we want 3 horns, we could write that down on the staffs, the samples for this idea have yet to come. We can approximate by going along with 4 being similar in sound. However the idea was another thing. We adapt all the time to this set of unwritten rules, fooling ourselves constantly into thinking we would deal with the real deal. So I could also argue, that a huge number of notes in your composition also could be at least recorded with metronome in a few takes of certain sections and maybe even performed at once with enough rehearsals. Yet you unconsciously put up with what samples gave you, putting up sample after sample what was already there. If van Gogh could have produced a brighter, more colorful yellow or a darker black, perhaps those would have substituted things that seem "perfect" to us. Didn't you think once that a more harsh staccato would have fitted that passage better? The evolution has still many fields to approach to allow for painter's freedom.
All the best,
PolarBear