I'll explain each one:

Orange Doodle: Mikey trying to cheer NiNi up by holding up Klunk.

Purple Doodle: Don holding a sign symbolizing "OK" because NiNi got a question right in studying for a degree.

Yellow Doodle: NiNi and NiChan. It's the whole mysterious effect about how they are the same in a way because of NiNi has NiChan's mindset.

Yep. That's it. Sorry for not coloring these or even commenting much. ^^;

NiNi/NiChan/Pic ©  NiChan
TMNT © Mirage Studios