I started one of these threads before
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IMHO he's actually ISFP, I first though INFP because of his idealism and anguish. If you look at his letters you will (hundreds available online) see a lot of his thought process circling the same material. Fi Se seems pretty obvious. Also just something of note is that Artists very focussed on texture seems to be ISFP that I have noted. [MENTION=1206]cascadeco[/MENTION] is a forum member (wonderful) ISFP artist who may be able to shed some further light.
I honestly don't feel qualified to type him, because I haven't studied all of his paintings or read his letters/memoirs as you have. Given that you're NeFi, though, and know Ne, I kind of trust that you'd recognize something that seemed fairly different from how you see things.
I can only speak for me personally, in that in my art, color, the medium itself, trying to learn as much as I can via the medium, improve, really bring the medium to life, is pretty important to me. As is painting what *I* find value in / what speaks to me. My subject matter though is not metaphorical, I'm not trying to convey some hidden or deeper meaning behind my subject matters. It's about color, aesthetics, the medium itself, and what I find beautiful/what I value.
I will say that my best paintings are when I'm really inspired and I just *know* how I need to paint. It just flows. My mediocre and bad paintings are when it feels more forced / when I don't have that flow / when I don't just 'know' what I need to do, where. Edit:I also think I have to be in the right emotional place to paint a particular subject, ie I'm not often in the mood to paint flowers, sometimes I am drawn to paint a shadowy forest, a lot of times I am not, and so on.
Re some of the quotes included in above posts, I share his affinity with nature; it feeds my soul. I also think I share more of his 'colorist' approach/style vs heavy focus on contrast and shadow.