What’s the etiquette on using diagrams that need color to be understood? I’m working on a paper that makes heavy use of colorful diagrams to supplement the text. For most of these it would probably not be possible to create grayscale versions that convey the same information as effectively. I’m a bit worried about this because (1) I imagine that some people like to print out papers to read them but these people might not want/be able to print in color, and (2) some readers may be colorblind.
What are the expectations on an author in my situation?  Will it be considered rude to leave as-is, so long as the diagrams are not technically necessary to verify the arguments?  Am I expected to include a description in the captions (“this region is red, this region is blue...”)?  Or most stringently, would I be expected to post a “colorblind version” somewhere that tries to recreate the diagrams in grayscale as best as I can?
I’m interested in all opinions, but especially those of people who would have trouble with color for whatever reason. 
 A: I can share my own experience. To make the diagrams with colored edges we use different line styles (dashed, dotted, bold, normal). We post the color version
in the arxiv. When people print it b/w, the style of the lines still can be distinguished. Of course we explain in the text writing something like this: red line (dashed)... .Some journals publish color pictures in online versions.
Few journals publish color pictures on paper.
For other types of diagram you can fill the regions with different filling style,
AND use color. The regions will differ both in color and in b/w print. 
A: The concern with color figures for grey scale printing is less of an issue these days, when pretty much all displays are in color, so the reader can always check which color is which even if the document is printed in grayscale. (Many journals no longer insist that the figures should display well when printed in grayscale.)
The issue with color blind readers is more substantial. Best practice ("etiquette") is to


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*Avoid ambiguous color combinations: green & brown, blue & purple, green & blue, light green & yellow, blue & grey, green & grey, green & black;

*Use a high enough contrast, which most color blind people can still distinguish.

*Use textures instead of / in addition to colors;

*Add a label or graphical element to distinguish different colors.


Here is a color-blind safe palette:

A: As a complement to Carlo Beenakker's very good answer, I suggest to have a look at the palettes proposed by Paul Tol. He purports to answer the very questions you asked. You can find his work summarised on his webpage, which includes a link to his detailed technical note on colour schemes.
A: Paper:   instead of one, let there be two identical diagrams except for different color codings.
Online:   one may have several color codings for the same diagram; only one at the time would show up on the screen--then click-click, and you switch from one encoding to another.
A: The top answer makes a substantial omission. Red-green color vision deficiency is not even considered (the most common one).
In this case blue and green are excellently distinguishable, but red and green not in every case. The color chart is not really easy to distinguish at all.
Also even if common, color blind is a misnomer, because usually people see those colors, just that certain shades have a higher contrast than others.
Colors that are not clearly distinguishable for "normal" vision, can be clearly distinguishable for people with so called color vision deficiency (such as more shades of yellow).
So, in real it's more of a preferred color depending on your way to view. "Normal" vision is not necessarily the more complete vision, just the commonly shared color palette most people perceive equally well.
The key to good color selection is to use high contrast, and avoid using non primary colors. This gives the best results. If you need more than 5 colors, you should use other ways to clearly distinguish information.
It should be clearly and easily distinguishable if you view it in grayscale. That works for everyone.
A: 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


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*style (solid, dotted, dashed, ...)

*thickness
objects


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*shape (circle, square, triangle, diamond, cross, ...)

*filled vs. unfilled


areas


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*pattern (plain, striped, dotted, checkered, ...)

*direction of pattern
