Is flavored coffee (black, no milk, sugar) okay the day before a colonoscopy?

So long as the beans have the flavor and you are not using an opaque flavor additive or creamer you will be fine. Flavored teas are fine as well. I would suggest you drink some sports drinks (no red, or purple because of the red food dye) as the prep is going to drain you of electrolytes unless you are taking one of the large volume PEG3350 based laxatives that also has an electrolyte added.