Is it possible that my contacts for astigmatism are making the whites of my eyes not as bright as before I switched to them?

Probably not. Soft and rigid gas permeable lenses shouldn't change the "color" or brightness of the sclera. Maybe if you're experiencing irritation where the whites of the eyes are maybe a bit reddish or inflamed this could give the impression of eyes less "bright". If your contacts don't feel right, please get checked by your ophthalmologist soon.
Inlammation. Contact lens wearers need to limit weartime and keep the eyes moist. A common inflammation problem with all contacts is known as gpc (giant papillary conjunctivitis), and is caused by overwear. This can cause the eyes to look more red, and less 'bright'.
Possibly. If there is any problem that makes your surface conjuctival blood vessels more prominent, then the whites of your eyes will not appear as bright. This could be related to issues with the cls. Please go back to the eyemd who originally fitted you with cls for astigmatism to have the fit checked. Dry eyes with different braqnds of cls can also make the eyes appear less bright.
Irritation from CL. The contacts wouldn't cause a permanent change in the color of your eye, but could be resulting in dry eye or irritation from the contacts which could make your eyes red, or less white. Make sure that you are not overusing your contacts, keep your eyes well lubricated, and if this continues see your eye dr as you may need to be refit for your contacts.