The chapter headings and some specific lines in the book are red font, which shows up fine in normal (white background) reading mode. But when toggling the kindle/e-reader to black or sepia background, the red font changes to white/brown along with the rest of the text. Any way to avoid this and maintain the red font regardless of the background? I'm using a very simple css for it:
p.red {color:#f00;}
Amazon may have resolved this for us, or it's just one of those things that worked itself out. I haven't changed the CSS at all after posting the question, but I DID convert the book to KFX format using the plugin for Calibre, and the renamed the KFX file with a .mobi extension, opened it in Kindle for Mac, and viola the colors are sticking: 
What you're seeing here is red font displayed in "night mode" - so I don't know if it's a factor of the "basic" mobi format (which would have changed this to white text) versus the KFX format (which is what Amazon delivers to a buyer through the Kindle store), but I would presume that this would be preserved through the KDP publishing process. Never a guarantee though.
PS sorry, I don't have time tonight to post how I went through the above process, but I can if someone needs it and leaves a comment.
To accomplish this, you would need to write a css media query supported in kindle which detects whether night mode is activated.
Here's a two year old thread about the topic. Not very useful, btw, and they go off on a tangent. They offer advice about how to create css media queries to target specific devices. This is not a good strategy for dealing with the issue.
You should download the latest Amazon Publishing Guidelines and maybe ask on the KDF technical formatting forum whether someone had stumbled upon a solution. I just looked there and found nothing.
In web browsers, you can make a css media query to detect night mode by matching color; ie, if color=black, then use this css color, etc. But that wouldn't work on a specific platform like Kindle, I'm guessing.
The thinking behind night mode is to provide complimentary colors. Its algorithm for determining what is complimentary is not documented yet, nor do I know a css media query for Kindle which you can use. It would require a good amount of testing to figure it out on your own, I'm guessing.
At the moment, I don't see an easy way to accomplish what you want -- though there probably is a need. That said, I do generally trust the algorithms in night mode which try to maximize contrast.
