Many scientific journals accept only black & white articles.
Does this mean that only monochrome (black and white only) articles or also grayscale (gray is OK) article are OK?
The same question holds for books.
I have a confession to make. I absolutely ignore it when a publication tells me they want grey-scale or black and white.
Yes, they want it because somebody is going to put something on some dead trees, they're going to be cheap about it and not use color, and somebody might actually pick up a dead tree and see my mangled image. But either they're also going to put it in a PDF, or else I'm going to put a pre-print in a PDF and that PDF is going to go online in all its radiant rainbow glory, and that is what people will actually read.
So [CENSORED] monochrome. That is so 20th century.
In general some degree of half-toning will be used, meaning that a limited set of grey scales can be output in the printed journal, so figures supplied in greyscale should come out in greyscale. Figures supplied in colour may (i) upset the editorial staff or software (extra hassle for you); (ii) be chargeable (to avoid this you may have to resubmit figures, extra hassle); (iii) reduce to greyscale rather inconsistently with what you expect. So if you submit colour figures (and I generally do), you may as well go for good clear distinctions between data sets, and print B&W yourself to check.
It is very possible (and in some cases required) to produce most figures in a way which either doesn't hinder the B&W reader too much, while still aiding the reader who works in colour. Two examples:
Both of these approaches are a small step towards helping colour-blind readers as well.
You can't assume your readers will work on screen -- an interesting paper may want to be annotated (I've not yet found a pdf-markup solution that comes close to pen&paper for this). Paper copies are also easier on public transport unless you have a very good and large tablet. It's also not uncommon for B&W printing to be easier (less far to go the the printer) and much cheaper (or uncounted and essentially free) than colour.
Much of this was going to be a comment presenting the opposite point of view to @jakebeal, but another couple of sentences made it an answer.
First, off: I am not sure that many only accept monochrome or B/W (usually implying grey-scale) figures since trends quite some time are for digital publication only (where colour or B/W, technically, is an irrelevant question).
The question about true Black and White versus grey scale is a matter of technique. In the analog days of printing grey-scale images had to be rasterized and this may have been a problem for some reproducing grey-scale illustrations. Today rasterization is done digitally within the printing process and should not pose any problem. It is therefore relatively safe to say that providing grey-scale illustrations will be fine. As I stated above B/W includes grey-scale nowadays and since quite a long time back.