Can you give us examples of any other fiction eBook that uses italics, bold and colour to differentiate headings within the book?
I don't spend much money on e-books, in fact I've only got four I bought. three use bold and larger font to designate the chapter headings, and that's all. The other uses colour and larger font for the chapter headings - and it's the most recent one I bought just a couple of months ago from G Younger.
However, I format my books different to most authors as well, and that adds to the situation. Most authors use only chapter headings and nothing else, while I use chapter, sub-chapter, and section headings. In the .odt version, pdf version, .html version, and SOL tagged text version I use colour to help identify them (note: in the SOL system the colour is the only way I can be sure of doing this). I need to differentiate between the three levels in a way that survives the e-book reader override attack code.
Some e-book readers will strip out and not display format code related to the e-book, regardless of how well you embed them. So you need to use something all the systems will allow through. Some will allow larger font only for what they recognise as chapter headings, thus they'll see the chapter and sub-chapter headings as the same and display them the same way.
However, there are some format commands that all e-book readers will recognise and apply, even when overriding embedded fonts (yes, some do that to you). The only ones I've found that will get through regardless are italics and bold because colours and font sizes will get stripped out by some readers.
Chapters are a larger font in red and normal style text. Sub-chapters are larger font in blue italic text. Sections are normal font in bold.
When the colours are allowed through it makes the story look nicer and helps designate what's what. When the colours get stripped out the difference between a chapter and sub-chapter heading is shown by the later being in italics. The section being in bold makes it clear it's a title and not some weird line of text.
It was this stripping of colour for some readers that lead me to use italics for the sub-chapter headings (I didn't before I found that problem).