The gist is that on the figure kits, you can customize their hair colors by painting the hair parts whatever color you like, and parts are available for a variety of hairstyles.

They are also making a cultural reference that Japanese minors dyeing their hair is associated with delinquents, while Ritsuka making her hair black seems to be the opposite (although her hair is naturally blonde).

HawkeyeNFO said:
The gist is that on the figure kits, you can customize their hair colors by painting the hair parts whatever color you like, and parts are available for a variety of hairstyles.

Kotobukiya actually sells versions of the kits with alternate hair and uniform colours. The blonde Koyomi and pink-haired Madoka are sold as separate (limited?) editions, whilst black hair parts for Ritsuka are apparently a limited pre-order bonus for the kit's re-release. Not sure where blue-haired Madoka comes in.