As Matt Grum has stated, you'll want to maximize exposure value (EV), but using the longest exposure at the widest aperture you can. Generally speaking, shorter focal lengths support longer exposure times, and when your goal is wide-field photography, focal lengths of 16mm or less are great (so your 10-22 @ 10mm-16mm will be an ideal lens.) To determine the exposure time you'll need to avoid star trails, you can use the "Rule of 600". This simply states:
Divide 600 by the 35mm-equivalent focal length to determine shutter
speed.
If you were to shoot at 10mm on the 7D, you would need to adjust to 35mm-equivalent (multiply by 1.62x), and divide 600 by that value:
600/(10*1.62) = 600/16.2 = 37 seconds
At 10mm, you could expose the sky for 37 seconds before you start to experience star trails. At 16mm you could expose for about 23 seconds, and at 22mm you could expose for about 17 seconds. At 37 seconds you should be able to get a decent shot with good color, so long as you use the right ISO setting. At all exposure times, I would recommend taking a series of as many photos as you can...dozens, even a hundred. A tool like DeepSkyStacker could then be used to align them, eliminate noise, and stack them into a high resolution (even super resolution) and high color-fidelity final image.
One area where my recommendation differs from Matt's is the ISO setting. I own a 7D myself, and at very high ISO's noise takes on a very different characteristic than at low and medium ISO settings. In particular, ISO 2000, and anything ISO 3200 and above, are really going to eat away at your color fidelity, while ISO 2500 and anything under ISO 2000 won't usually affect color fidelity in any seriously adverse way. It is unnecessary to use that high of an ISO setting with a Canon anyway, as read noise drops to its normative low by ISO 400, between 2.8 and 3.4 electrons worth, vs. 8.6e- at ISO 100 and 4.7e- at ISO 200). The ideal ISO level from a read noise and color fidelity standpoint would actually be ISO 800, where it is only 2.8e- (see SensorGen Canon 7D for details), although with the higher maximum saturation of ISO 400, the 3.3e- read noise won't be a major problem.
You will probably want to tune your ISO setting, which affects the maximum saturation level, to the exposure time. A longer exposure will be more capable of saturating more, so at ISO 400 you could use 10mm and a 37second exposure so a higher max sat. level is useful. For a shorter exposure, such as at 22mm, you might want to bump ISO up to 800 to allow the exposure to saturate faster, and maximize SNR, etc. Keep in mind, the higher the ISO setting/shorter exposure you use, the more the random physical nature of light will affect your shots. At ISO 1600 or higher, with very short exposures of only a few seconds, regardless of whether you manage to saturate some pixels, a lot of pixels will likely never receive any light at all. At this point, taking as many shots as you can and stacking them in post will be the only way to really create a "complete" picture with high saturation and good color fidelity.
