Here is some special information on color figures provided by Carolyn Chmiel of the University of Chicago Press Production Office. (I have made some small changes for clarity.)  For the latest information, please check http://www.journals.uchicago.edu/ApJ/instruct.html.
 

Color Files: RGB vs. CMYK

The UC Press production staff fields many questions regarding the use of color files in the printed and on-line journals. The primary problem is that most computer-generated figure files are created using the RGB (red, green, blue) color model, and we prefer CMYK (cyan, magenta, yellow, black) files for the printing process. As of 2001 May, RGB files can be accepted as well, but color shifts may occur. (See below.)

The conventional printing process uses the CMYK system with printing plates created using film negatives or electronic PostScript files. Each of four printing plates is inked with one of the four process colors, and the ink is transferred to paper in layers.

The RGB color model is used for devices, such as computer monitors, that create color with light. The available RGB color gamut is much larger than the gamut for CMYK; in other words, there are colors that look fine on an RGB monitor but cannot be printed. The major culprits are brighter colors like deep green, blue-violet, and orange-pink. Color figures prepared as RGB EPS files can be converted to CMYK, but because the available color spectrum is much diminished, it is very difficult, and sometimes impossible, to match the colors between the two formats. There is currently no technology that allows an RGB file to be printed without first converting that file to a CMYK format. Once a channeled CMYK EPS file is created, it can be used to create the four printing plates needed to reproduce an image.

Hard copies produced from RGB files by desktop color printers can still contain colors outside of the range of the CMYK palette. Desktop printers can use dyes or wax transfers that create colors not able to be duplicated by the CMYK color gamut.

We are able to use RGB files in the on-line journals, and we encourage authors to explore this option. Figures can appear in black and white in print but in color in the electronic journal. The color files are still device dependent and may not appear on the reader's monitor exactly as they do on the author's monitor, but some authors may prefer this option to scanning a hard copy or converting the file for printing. Authors should still pay attention to how the figures will look in the printed Journal. The printed black and white figures should indicate to readers the nature of the information shown by the color figure, even if the information itself requres viewing the color figure. In other words, a grey smudge in the printed version is of no value and should be avoided.