Because there arent icons of bags of different colors.
However, icons could be made, though dying bags (essentially swapping out one bag for a different one) would require either a script, or the addition of different colored bags into the crafting system (more CNR recepie entrys.)
Also, any script to change the bags color would ignore the bags contents, so, if you used it when the bag had anything in it, you would lose the contents.
its an interesting idea, but I think the only fesible way to implement it (without being overly work intensive to make and having significant drawbacks) would be to give the crafter the choice between different colored bags when she crafts it.
If its chosen to implement this lal and I will make colored bag icons