Hopefully she is just a little slower to mature, which happens, but I'm going to share the following just for info just in case.
It's possible that she has a heart problem. The darkening when she runs or gets agitated could indicate lack of oxygen, which suggests that her heart maybe isn't up to the task.
I recently lost one to heart failure at 7 months old. Same symptoms, pale and underdeveloped comb and wattles, she never laid, her comb and wattles would turn darker/bluish/purple under stress or activity. She stopped running at all, only walked slowly. She grew and feathered out normally, was a good weight, not fat, no other signs of illness at all. I did suspect that she had a heart problem, and could feel her heart racing and beating through her chest when holding her. I did necropsy her when she passed and she had an enormously enlarged heart that was very soft, probably a miracle that she made it 7 months. I also noticed that she had more trouble in warmer weather. I hope that is not what is going on, but some things to look for, in case.