[pdftex] color -> grayscale switch?

[Taro Sato]

Hello.  I have a question.

Is there an easy (or just standard) way with pdflatex to do a
conversion from color to grayscale when a PDF file is generated?

Ideally I want to come up a scheme where after a document is prepared
assuming full color, I wish to have an option of generating PDF files
in grayscale via adding only a few lines to the source or via a
command switch, if exists (not likely).

Any help would be appreciated.

Taro