While some are mislead by the fact that Inkscape does not (and probably should not) support raw or "generic" CMYK, it does in fact support working with true CMYK for print support. The key factor is that Inkscape only supports real CMYK work, and not "pretend CMYK." In and of itself "CMYK color" does not mean anything specific. It turns out that "RGB color" is also meaningless as far as specifying an actual color goes, but the variances are usually not as strong. To get accurate RGB color, one needs to specify *which* RGB to use. SMTP television values, Adobe RGB, Wide Gamut RGB, sRGB, etc. For experience on the Internet, people usually don't realize that there is an implied colorspace of "sRGB" used for tools, browsers, etc.
In a similar manner, Inkscape needs to be given a *specific* CMYK colorspace to work in. In fact, Inkscape (and SVG itself) can support a document with several different colorspaces at once, including mixing multiple different CMYK colorspaces alongside RGB colorspaces. This could be useful for cases such as when a graphic designer is creating artwork for some brochure that will be printed with cutaways and different paper types. Or it could apply to a case where different printers are used for different parts of a job. (Figure 1 shows what appears to be the same colors)
However an fairly common use case is where one might create a document to be printed mainly in CMYK, but with one or more spot colors, such as Pantone, Toyo, HKS, etc. colors. In this case, some elements of the artwork can be marked with a specific target CMYK profile, while the elements to be done in a spot color can be specified with a named color profile supporting the type of spot color (for SVG 1.2) or perhaps even with simple sRGB equivalents. Then when things go to a service bureau to be run the CMYK elements can go to a four-color print and the spot colors can go to custom plates per ink. (Figure 2 shows that the colors are actually different)
So how does only get to "real CMYK" in Inkscape? It's actually fairly simple. First at least one CMYK profile needs to be added to the document being worked on. That can be accessed in the GUI through the "Color Management" tab on the document properties dialog. Once at least one profile has been added, the color pickers in the Fill & Stroke dialog can be used to pick colors in that colorspace via specifying the ICC profile. In the past one needed to use the CMS color picker, but with Inkscape 0.48 the other color pickers such as the "CMYK" one will attempt to preserve values. (Figure 3 shows that the exact same CMYK numbers were used)
The main problem left is that even though true CMYK values are stored in the saved SVG value, using those values is now a bottleneck. Printing directly from Inkscape will flatten things to sRGB, and even PDF export will not yet preserve the CMYK values However, other software can read and use those values. Recent versions of Scribus will read in and preserve ICC colors including CMYK, and will happily save high-quality print-ready PDF output. (Figure 4 reveals the difference in RGB and visible colors that result from using two different CMYK profiles)