[Gimp-user] colour to black and white

[norman]

I would like to be able to produce a black and white negative image from
an ordinary colour image. The effect I would like to produce would be to
imitate what one would get if you took a colour, positive transparency
and made a contact print on to either orthographic or lithographic film.
Thus, blue would be black and red would be clear and transparent. Have
you any suggestions how to approach this using the Gimp, please.

Norman