Hi,
I'd like to add few things to Nikodemus' list:
- to protect the item (weather, bacteria, spying eyes...)
- to keep items organized
There could be different colours of dyes to choose from. Players could colour their sacks to which ever hues they prefer, this way the colour blind persons can choose colours that they can most easily distinguish. The dyes could also be used multiple times on one item, to produce different lightness values (light, medium, dark), so you wouldn't have to rely completely on colour.
Personally I don't like the idea of seeing a sacks contents without opening the sack, it's just too automated for my taste. I think it adds a nice touch of realism that if your containers are poorly organized, you'll have to search through them to find what you seek. If you keep your inventory tidy though, you'll know exactly what you put where.