This group is the most difficult to categorize. Women who fall into this category choose their clothes for their simplicity, classic style and modesty. They seek to express their religious convictions through what they wear. Skirts and jumpers are popular, but trousers for women tend to be perfectly acceptable. Least likely of my religiously observant dress designations to choose to cover. Women in this group are most likely to have shorter hair styles, as shorter can be simple and easy to care for. Makeup is often avoided as a statement on simplicity, but jewelry is common, particularly earrings. Actual plainness (solid colors, simple print, limited-palette wardrobes) is a hallmark of the plain modern look. There are neither anachronisms nor symbolically or traditionally important items.
Some plain modern resources:
I am not Amish or Mennonite, but some people who come to my website are interested in knowing more about these groups. I can recommend these books as authoritative and relatively inexpensive sources of further information.