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**:

1. there are no issues for people with colour vision deficiency

2. many journals do not print in colour (unless you are prepared to [pay for colour printing](http://cofactorscience.com/blog/author-charges)), so library copies will usually be black & white

3. 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*