Many journals require color figures to be in CMYK format. If you are not doing this over and over again, here is a quick and dirty way to do it, assuming that you are a Windows user.
1. Create your RGB figure, say, foo.tif.
2. Download and install ImageMagick from http://www.imagemagick.org/download/binaries/ImageMagick-6.7.7-7-Q16-windows-dll.exe
3. Download Adobe CMYK profiles from this link http://www.adobe.com/support/downloads/detail.jsp?ftpID=3680
4. Copy the profile files (with extension .icc) to the folder containing your figures.
5. Run the dos command from that the figure folder
convert foo.tif +profile icc -profile sRGB.icc -profile WebCoatedFORGRA28.icc foo-cmyk.tif
Of course, the colors in the CMYK file will look slightly different from the original RGB file. Try use a different icc file in the command to get the best result.