The dark cherry barbs are the males and the lighter ones are females. It is the males that are the nice red colour, where the females are somewhat dull...
It is recommended that you keep 1 male and several females. Apparently the males can be aggressive towards each other and there is a chance they will fight. As for my experience, when I first bought cherry barbs, I bought 6males and never had any problems with them fighting, I had them for about 3 years. At the time I did not realize I had only males. They were also housed in a 3foot 35 gallon tank with some tiger/gold barbs... Like I said, never had any problems with them. Maybe it was because they had enough space, or maybe because there was so many of them they didn't fight. Not exactly sure why, but they didn't and everything I read says they will.
Your best bet is to get a few females and a male, or if you like the males like I did, get lots of them and see how you make out.
Either way, good luck. They are nice little fish, hope you enjoy them
Colen.
EDIT: Forgot to mention that it is only the amales that fight with each other (not in my case though). Any the females wont fight.