One of my favorite colors in the whole world is the iridescent green-black of a mallard duck's head. I've looked for a nail polish like that - and I think I've found it in Finger Paints "Avant Garde Green".
This is an incredible color, a shimmery emerald green, that looks almost black at oblique angles in direct light. Unfortunately, I had trouble with application... the Finger Paints brush is a standard narrow round, and the formula is on the thick side, so the brush doesn't fan out well, and the polish doesn't flow well on the nail. I ended up putting it on too thick to compensate, which led to bubbling. I had to redo several fingers, and one needed redone again... On the bright side, it does seem to dry faster than other Finger Paints I have used. Still, the result is worth the effort - I love the color, and I'll wear this often!
Just for fun, I tried another approach to achieve the same "Mallard Green" effect.
This time, I applied an opaque, true black. Then, I floated two drops (per 2 nails) of China Glaze "Watermelon Rind" on water, and dipped my nails as per water marbling. This gave me a very thin, even coat of WR over the black. WR is more sparkly than shimmery, and at this thickness it flashes brilliant green and then abruptly goes black. The color is darker than WR on its own (shown on the pinky), and is very similar to the color of the AGG above... but the color is more "active" in motion (makes it harder to photograph though), and much more dazzling in sunlight. Plus, even with the water dip, application was easier because the black goes on super smooth and glossy, the green lays right on, it all dries faster, and I had no bubbling issues and no need to redo anything.
Of the two versions here, I like the second one better!
(Disclosure: I bought this all myself, I have not been compensated in any way for this review.)