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Try adding -fuzz 15% to your command line. Fuzz permits close colors to match along with your target color.
The fuzz option won't help with this image. If you magnify it with an image viewer you will find that there is a margin along the top and left sides of the image that is two pixels wide consisting of various shades of grey pixels.
This is a screen grab of the top left corner of the image (I used Paint Shop Pro).
I think trim will work if you first crop that margin out of the image.
You could also try -trim twice (e.g. -fuzz 15% -trim -trim).
I've played a bit with cropping and trimming that image and the margins are a bit more awkward than I thought.
In order for trim to work you have to crop a two pixel margin off the top, 5 pixels off the left margin and 5 off the bottom margin and then trim (with a fuzz of 15) reduces the image to 899x2269 from its original 982x4000.
for more information on -trim see
http://www.cit.gu.edu.au/~anthony/graph ... crop/#trim
This includes fuzzy trims, trims with a specific color, and trim ignoring small features.
If anyone likes to add other techniques, please mail me.