First: You probably have to update Ghostscript. I'm using current versions (Ghostscript 8.71, ImageMagick 6.6.1-5) and have better results/colors (with a simple "convert -density 300 test.pdf[0] output.png" command):
http://www.imagebanana.com/img/m84z7a2n ... dpi_im.png
But: There is still a difference to the Adobe output. (Adobe Reader shows
that for me.) Sorry, I don't know what causes this..
One can see that the PDF has some transparency values in it - the 21 red squares go from fully opaque to fully transparent (1.0 to 0.0, with 0.05 steps; the 8th value seems to be off, it's 0.62 instead of 0.65 there). I can recreate the transparency with ImageMagick (without the PDF) and do get the same red tones as Adobe.. So I guess the Adobe output is the correct one.
ImageMagick has no fault here. You'll probably have to take this to the Ghostscript site if you want to resolve it.