Docs for PixelSetMagentaQuantum() say:
The green magenta.
which sounds impressively psychedelic.
Also, all of those functions return unsigned ints, but don't say what the return value means.
The problem which is specific to PixelSetMagentaQuantum() is fixed via Mercurial changeset 14087:a9cdf0521b82. The return status issue is a general one which will need to be addressed later. The return strategy used is that a return value of True (1) indicates success while a return value of False (0) indicates failure. This is made more clear in the rest of GraphicsMagick by declaring the return status as type MagickPassFail with results of MagickPass and MagickFail. The Wand API should be updated to do the same.