As with all digital graphics, you need to ensure that your monitor is correctly calibrated before you can start. Use the Adobe Gamma utility to create an ICC-compatible profile. You can check that this is properly installed by accessing your graphic card's Properties/Advanced/Colour Management section.
For absolute colour fidelity, no system based on visual observations (such as Adobe Gamma) can match the accuracy resulting from precise electronic measurements using a proper calibration utility and photospectrometer.
Illustrator supports more than one colour management engine. Its default might the best all-rounder, but cross-platform users may prefer the Kodak Digital Science engine.