Actually, not being so complex that I actually need to think is what I really like about 0hh1.com.

It requires a non-zero but pretty low level of "executive function", but significantly less than most logic puzzles.

In particular, a few times per game, I have to apply a little bit of "frontal-lobe" or higher-level skill to prevent myself from losing my patience for constantly applying the lower-level or "perceptual" skills required by 0hh1. In particular, I have to apply frontal-lobe skills to resist the temptation to engage in the "wishful thinking" of falsely believing I have perceived a correct next move to make.

(I engage in 0hh1.com mostly to relax or unwind, not to hone any higher-level skill.)

For me it was somewhat (but not exactly) the opposite: I encountered 0h n0 first, and was quickly able to finish each level without the computer hints, while I currently still need the hints sometimes for 0h h1.

It seems 0h h1 has a slightly harder learning curve before the pattern recognition kicks in, while 0h n0 doesn't rely as much on pattern recognition, but is easier to learn.

Is every level solvable without guessing? Or are there multiple solutions for some levels? I feel like I had to guess sometimes, but maybe I haven't thought about it enough.

I have yet to come across a level that requires guessing. They definitely exist in theory though(uncertain whether the level generation algorithm is smart enough to eliminate them).

There are no multiple solutions due to an unstated constraint that resolves ambiguities. I believe it is that a blue cannot be surrounded completely by reds.