Yeah you definitely need more cover in your sorority.
I have mine so that when I look in from the top, I can barely see the substrate. If you're comfortable with using live plants, I find it a cheaper and easier way to fill up my sororities. Plus if your plants grow well enough, you can always sell the trimmings.
Out of all my bettas only one or two will regularly use hiding places such as terracotta pots and PVC tunnels. I've found cover, particularly that offered by floating plants, to be much more important than lower level hiding places.
Because your females are in constant visual contact with each other, the more submissive ones are going to try and make themselves as unappealing target as possible. This is why some of them are that dull brown colour.
Hmm... Well the four girls get along. O_o however they are siblings... When Madame was spawning, all of them were super vibrant (fins) and gaining some color in their fins.
I have floating plant in there that's super fast growing >< I'll take some of it in sections and can attach it to the bottom, walls etc :) plus I really want Madame to be moved! D: I know she killed my snail, and pretty sure she played a part in one of my other fish's death >.> she's become more mean since I've gotten her. I just have to figure out where to put her...
What are some good tall (live) plants for a 20 gallon?
For a sorority you want the fastest growing plants you can get.
In my tanks I've found Water Sprite, Wisteria, Hornwort (it does drop its leaves just to spite you lol) Guppy Grass and Ambulia to all be easy-care, fast growing plants.
Plus Water Sprite, Wisteria and Ambulia can all send off emersed growth once they reach the surface so the roots provide somewhere for your bettas to shelter. Frogbit or Dwarf Water Lettuce are also good as they both have long roots that can offer protection to the lower ranking females when they go up to breathe.