The default is a light yellow which i don’t like. I’m using LibreOffice 4.
No, currently you can’t change the background color of comments in Writer.
You may file a request for enhancement on Bugzilla (Severity: enhancement).
From OOo-Wiki · Notes2 OtherIdeas, 2008-10-13:
Briefly: User’s preferred notes window color.
In details: In extension to the “Same Colors for The Same Authors” idea, it could be interesting for a user to be able to chose his/her prefered notes window color in the OOo options (among the 9 currently available colors). Therefore, whatever authors edited the document and in whatever sequence, and independently of the document being opened, the user could set that his/her notes should always be displayed in the same color. […]
Memo: I added this idea without creating an enhancement issue, nor speaking about this with people, nor thinking about the technical issues it could lead to. The goal is to write the idea down, so it would not be lost. […]
See also: OOo Wiki · Notes2 Design MainColors
Request for enhancement → Bug 61242 - Customise comments background color in Writer
Correction link: OOo-Wiki · Notes2 OtherIdeas