Which RGB mode are you working in?
This is not something we necessarily think about, but for printing this is helpful to know.
There are two main types of RGB modes: sRGB and Adobe RGB. Most, if not all, compact cameras shoot images as sRGB. But some, including high-end DSLRs, can also shoot in Adobe RGB mode.
The difference? Well, Adobe RGB is best suited if you plan on converting images to CMYK at a later stage for printing. Changes in colour are less dramatic than they would be if converting from sRGB to CMYK.
sRGB is best for uploading to the web, due to its small (that’s the ‘s’ part) colour space.
Photoshop doesn’t tell you which one you’re working in inside Image>Mode; it just states RGB Mode. Go to Edit>Color Spaces and look inside Working Spaces for Photoshop’s specific RGB setup.