The supply and delivery missions unlock at Sgt, rank IIRC.

Every expansion the lowest level scrips, currently blue, go away. The middle level, currently red, become the lowest and also become useless except for trading for the former highest level scrips, currently yellow. The yellow scrips become the middle level and a new color becomes the highest level.

Once you get to L55 gathering you will will be going for the yellow scrips, but nothing else about the process changes.