Grain is intrinsic to film, but it becomes prominent with: high-ISO stocks (800+), push processing, and above all underexposure. When a thin negative is brightened during scanning, shadow grain blows up. If a roll looks "rougher than expected," underexposure is the usual culprit.
Color shifts typically come from: (1) expired or heat-damaged film, (2) underexposure (negatives store little color information in the shadows), (3) unusual light sources (fluorescent, mixed light). The classic case: tungsten-balanced cinema film (500T) shot in daylight without a filter goes blue.
We adjust color and density frame by frame, but no scan can recover information the negative never recorded. "When in doubt, overexpose negatives slightly" is the single best rule for clean scans.