Even though I'm only supposed to drink clear liquids before my colonoscopy, can I have a few glasses of soy milk?

Not a good idea. There is no way to make a bowel prep for colonoscopy sound fun or glamorous. Havingf said that, if you are going to do this, you want the best prep possible so your colonoscopy can give you the most information. No one knows if one glass of soy milk will still allow an optimal prep, or two glasses, or three... So stick with the clear liquids for your prep. Soy milk tomorrow!