How to convert color to black & white

Dear all,
Could someone please explain to me how to convert color to black & white.
For example:
barplot(1:5,col = rainbow(5))
Because I need to print my plot to save my ink color printer.
I don't want to convert to grayscale, but keep it as an RGB.
I  would be very happy if anyone could help me.
Thank you very much in advance.

Kindly regards,
Muhammad Subianto