What color space are you using? CMYK (or any of the CMYK "flavors")? Apple RGB? sRGB? Adobe RGB? Colormatch RGB? ProPhoto RGB?
Each color space will give different colors and/or saturation (sometimes significantly different) depending on what color space you use, even if you feed the same RGB values into them. To see what color space you're using, go to Edit > Color Settings... > Working Spaces. I have mine set to sRGB which, though it doesn't have as wide a color gamut as Adobe RGB or most of the other RGB spaces, is used by the two online photo printers I use, so I leave it on that setting. You can have Photoshop warn you when a picture you're importing does not match your preferred color space, and choose to have Photoshop change it automatically or ask you what you want to do. I like knowing exactly what I'm working with, so I have Photoshop warn me, then ask what I'd like to do.
You may want to do a Google search on "Color Space" to learn more about this. Even if the cause of your problem is not color-space related, it is still good to know about if you plan to work with color.