As for Yumichika, he explained it in Soul Society arc, thouhg you guessed it pretty much to point.
Everyone in 11th division uses fighthting-style Zanpakuto and Kido-based ones are looked down upon. That's why Yumichika nicknamed his blade with different coulour, which his blade doesn't like - Fuji Kujaku (Wisteria Peacock), cutting off Kido-based power. When he's using true nake - Ruri'iro Kujaku (Azure Peacock), it releases fully.