I like it; I don't have a device to test it, but here's some thoughts based on the presentation here.
Graphics-wise, you're off to a decent start. I'd suggest larger circles, rather than shrinking the font size when the numbers don't fit. They end up a bit off-center. And maybe have a secondary indication that a circle is zeroed out, like having it light up a bit more, or inverting it, making it a black circle with a white zero. (That helps as a tutorial; it's natural to think "Oh, I've gotta light all of these up."
For colors, you might want to think about choosing more similar shades but with different intensities, rather than four different shades of about the same intensity. If you look at the game in monochrome, you can see that your four colors are around the same saturation. It's a bit more tiring for the eyes and brain to process.
![](http://www.color-blindness.com/coblis/uploaded_images/Screenshot%20(6)_achromatopsia.jpg)
I'm not great with color either so I don't know what colors might work for you, but if you go with different colors here's an easy trick for getting a palette that works together:
http://forums.tigsource.com/index.php?topic=25396.msg839374#msg839374It might be worth considering having a bit of texture in the background, just something subtle. (Like you don't notice the texture in the background on this site, but it still gives it a bit of a tactile feel.)
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For the tutorial, I don't think I understand what it's trying to say, like why the -1 changes to +1 but doesn't change the 1 or 2 at all. I think it's enough if you just show them the right way, the wrong way just confuses things.
Or no tutorial at all, just (1) (2) (1) and then ( -1 ) ( -1 ) as the first level. So that the only obvious thing to do (drag loops onto circles) is also the right thing to do. And then maybe for the second level give an easy choice, like (-3) (-2) (1) and then ( +3 ) ( -1 ). So there's one right answer and one wrong answer, so you either get the right answer, or if you don't there's only one other thing you can try.
Anyway, good luck!