I agree with Misha.
That is normal behavior for young birds. Even hens will chase and play fight with each other. Then can puff the short neck feathers and throw feet at each other. Just like how roosters throw feet trying to spur & chase other males.
Darker is like you thought, the rooster/cockerel
and light brown is the hen/pullet. They are a lovely pair.
Roosters have thin long neck feathers, usually with more shine to them.Their tail feathers are also longer than females as they mature. Noticeably they have shiny feathers as well, commonly in darker plumage like yours, you will see either shiny green or black with a hue of purple blue or even a red cast as light hits these. Leg bones will develop thicker, you can also see where the spur is developing. If not catch him and running your hand over his leg and notice small knub, the formation of the spur.
Hens have the wider neck feathers and short tails, with no various colors other than those on their body. Their legs are thinner and can have a small spur knub as well, but it's much smaller and will not develop further.Edited by TalkBahk2Me - 8/30/16 at 1:31am