Check out this website. It is a great resource for all things minecraft. Link is to how to use color in books. Formatting codes – Official Minecraft Wiki
Just like in chat, 1-10, a-f. Only that instead of using & before them, you use § So, §a-f or §1-0 works exactly like &a-f / &1-0
This is how to do it, just an extra bit of info: I'm pretty sure you can't type the § symbol and you have to copy-paste it into the book Has your question been answered sufficiently?
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