Some information about the BGM composers
http://www.exelion.jp/
Wow, EXELION composed part of the BGM of "Koiiro Soramoyou"
http://fairy-sound.sentive.biz/works.htm
Sei from
SENTIVE also composed some of both "Koiiro Soramoyou" and "Koiiro Soramoyou after happiness and extra hearts"
Maybe there're more?