Aperture uses on-screen proofing based on the colour profile you tell it to use. iPhoto doesn't. Have you got an accurate colour profile for your printer? Are you using the correct profile for your on-screen proofing or are you proofing using the screen profile?
Aperture uses on-screen proofing based on the colour profile you tell it to use. iPhoto doesn't. Have you got an accurate colour profile for your printer? Are you using the correct profile for your on-screen proofing or are you proofing using the screen profile?
Basically you can't. Aperture is a professional app that is designed to be used in a pro setting: you are meant to have a colour profile.
You can turn off onscreen proofing (if it's turned on). It's in the view menu, right at the bottom. The best thing to do would be to get a profile for the printer, install that and then set the onscreen proofing profile to that: then Aperture will do it's best to show you accurately on screen what the print will look like...