Why did Kaoru's hair color change?
When they got married, Kaoru's hair was black.
After a while, Kaoru dyed her hair blonde.

In the web comic version, chapter 6 has the story explaining the reason why Kaoru dyed her hair blonde:



Title: Sadako


1st frame: (With impersonations of Sadako) I—am—Sa—da—ko—
2nd frame: Hajime: Uwah~~~ Sadako-tan so moe~~~ I want to lick you all over~~~
3rd frame: (Kaoru is angry)
4th frame: Hajime: Oh? You dyed your hair? (Kaoru is sulking)


I think, Kaoru did a (light) cosplay of Sadako to get Hajime's attention, but he called her Sadako instead of Kaoru, so she got angry and dyed her hair blonde.