There is no option to change the color for the rules and the only option
you have is to use color conversion.
\setupcolors[cmyk=yes,rgb=no]
I do have this setup, but it produces grayscale crop marks.
I was told by two different printshops that grayscale raises problems
with recent versions of PDF software (verification? imposition? I've not
asked), that complains about that, because it treats grayscale as RGB,
splitting it on C, M and Y channels, instead of mapping it only to the K
channel.
If you need more control you can create your own marks and add them as
background for the page.
I've seen there's something about that in the mailing list history. I'll
look into that.
BTW, the MP file producing the marks is
metapost/context/base/mpiv/mp-crop.mpiv
Massi
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