With colour negative overexposing will decrease your saturation. The amount of opposing colour will increase more than subject colour density, thus less separation and also less contrast.

Spot meter whatever you want to have maximum saturation. For sunset/sunrise, this means severely underexposed foregrounds, just like slide film or digital, so you'd use a gradual ND or gradual reverse ND anyway, which you should be doing anyway to get the high pictorial contrast unless you want flat pictorial contrast.

Some films like 160S don't respond well to sunset colours, low saturation/low contrast doesn't explain failure to reproduce colour, it still should reproduce the colour, avoid those.