Of course going through existing coloured diagrams and redesigning them in a monochrome (or grey-scale) colour scheme would be a lot of work, and depending on the complexity of the diagram the diagram may actually benefit from the use of colour.
But just because no other answer has mentioned the benefits of black & white (or greyscale) diagrams:
there are no issues for people with colour vision deficiency
many journals do not print in colour (unless you are prepared to pay for colour printing), so library copies will usually be black & white
some researchers will print the electronic version (or view it on an ebook reader) and having diagrams compatible with standard black & white laser printers is very convenient (not only for the printed journal copy).
Even though colours can be helpful, every diagram can be redesigned in black & white and there are several techniques for distinguishing elements as you would with colour:
lines
- style (solid, dotted, dashed, ...)
- thickness
objects
- shape (circle, square, triangle, diamond, cross, ...)
- filled vs. unfilled
areas
- pattern (plain, striped, dotted, checkered, ...)
- direction of pattern