Maybe ThatFishThough means that you should be aware of the colors. Sometimes bettas will attack each other due to them thinking that the other is prettier than the her. I have read that on many sites but it rarely happens. I have only had one betta fight in the 4+ months that I have had my sorority and my friend has one too and has only had a few small fights. The only reason that mine fought was because I had a baby in the tank and the oldest one picked on her and bullied her due to her size. I don't think that you would have an issue with colors though. I would be more concerned with sizes. I "rescued" two smaller bettas (I'm assuming they are babies) from petsmart and they seemed to only want to be with each other. The only reason I took them in was because I didn't want someone to take them and put them in a bowl and have them get stunted when they are so young. I have one really aggressive girl that I had to remove from the tank a few days ago. She wrestled one of my babies to the bottom of the tank and injured her very badly. I had to remove both of them from the tank and treat the hurt one. I put the bully in her own tank where she will remain. She is far to agressive with other fish. As for the injured one, she passed this morning from not being able to move her fins. Here are some tips that I have for sorority owners:
1.) Make sure that you keep them in correct numbers. I would say 9 or 10 for 20 gallons and 5 for ten gallons. I would really keep 9 in a 20 gallon just so that you have space. I do have to admit that I only have seven girls in my tank due to a fight resulting in one death and a decision to take out an aggressive girl. I will hopefully be adding some more girls soon. I got lucky enough to still have an odd number which is good.
2.) Make sure that the girls are all the same size! Baby bettas may be cute but stay away from them for sororities! They will most likely get beat up and end up dying like mine did!
3.) Add LOTS of plants! It doesn't matter if they are fake or live just make sure they have lots of plants to swim through and hide in! It really does help out a lot! If you are out a lot and they start to fight the damage will be less extravagant if there are places to hide. Fights don't happen often but its still good to be prepared!
4.) Add some entertainment to the tank. I have added some floating tunnels and some small spots that they can swim through and hide in. This will keep them from even thinking about fighting. I haven't used this trick myself but some people use terricotta pots in their tanks. You want to make sure that you clean it just like any other decoration though. If the hole at the bottom of the pot is smaller than your fish you might want to seal it with some aquarium sealant to make sure no one gets stuck when you aren't looking.
5.) Have extra containers, kritter keepers or something on hand in case someone gets in a fight or you decide that one of the girls is not fit to be in a sorority. I wish that I knew this tip before because I am one year away from being able to legally get a job which drives me crazy. I don't have a steady cash flow going into my piggy bank so I sometimes have to rely on my mom which doesn't always make her happy lol. I ended up having to use an old fish bowl and borrow my little brothers extra heat lamp for his hermit crabs to heat one of my injured bettas up. In short, HAVE EXTRA SUPPLIES ON HAND!!!!!
I hope that this helped! Some of these tips were simply learned through other people and some of them were just from trial and error. Having a tank is always hard but this type of tank is ecspecially hard because bettas can be super unpredictable when put together. I hope that you enjoy your tank!
(P.S I forgot to add this tip: Always be on the watch for ich and other parasites! They seem to visit community tanks more often! I ended up having to make a few emergency runs to the pet store in between school and training sessions with my off-site show animals (I show livestock. Mostly pigs, goats and rabbits but I also do other animals sometimes.) to get ich guard and other medicines. Make sure that you have ich guard, aquarium salt, melafix and stress coat on hand at all times! It really helps out when your in a pinch ecspecially if you are as busy as I am! I reccomend having a small bag, basket or shelf dedicated to fish stuff. This way you can keep your fish pharmaceuticals in one place. Goodluck with your new fish!!!!!)