The missionary dress code has changed a lot over the years to stay compatible with current trends and styles (without compromising modesty, of course).
Until about 2010, sister missionaries were required to wear nylons and dark dresses/skirts that went down to at least mid-calf, as well as dark suit/jacket tops. In 2010, the dress code was updated, permitting:
- Knee-length skirts instead of always down to mid-calf
- Nylons not required
- Bright, vivid colors
From this news report, an MTC employee said:
"Having recently served a mission, I knew how people reacted to the floor-length skirts and generic collared shirts," Porter said. "Sure, we looked professional, but not all that current and approachable. Being a part of the standards update ignited my excitement for missionary work. Now sisters could feel cute, comfortable and still modest. And that was the goal.
"Everyone working on the project had the same collective vision," she continued. "We wanted the sister missionaries to be more approachable, more up to date with fashion and as always, modest."
Basically, the change now allows sisters to look more natural, approachable and personable. It allows their personalities to shine through a little more, and that's something the LDS Church considers valuable when sharing the gospel.
Similarly, elders are now allowed to wear light-colored suits and slacks. Before 2011 or 2012, all their clothing (except the white shirts) were dark. To answer your last question about ties and suits, the lack of variation might just be because few men straight out of high school have good fashion sense. (I jest, but only a little.) Dark suits are still most common among elders, though, it seems.