Twenty thirteen different background colour per page

What exactly are your extra pages? Do you mean under Pages –> add new?

1. Are you using a child theme? This is critical ?? https://codex.www.ads-software.com/Child_Themes

2. After you have set up your child theme you’ll want to edit the colors in the child theme style.css.

3. To find the colors go and look at your dashboard Posts -> Add New. On the right hand side there is a list of post formats: standard, image, chat, etc. Write those down somewhere handy.

4. In the style.css sheet scroll through until you see “5.5 Post Formats.” Below that you’ll find each of the post formats that were on the “Add Post” page.

5. The first one under 5.5 Post Formats is “Aside.” The style directly below looks like this:

/* Aside */
.format-aside {
	background-color: #f7f5e7;
}

6. #f7f5e7 is the color code for the background for the Aside Post Format. Don’t worry, you don’t need to know or memorize these numbers, there are websites that you can plug it into and see what the color is: https://www.color-hex.com/color/f7f5e7

**It’s very important that you make all of these changes in a child theme or they will be lost the next time you upgrade.

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The link below will take you to a nice write up about how to set a specific color for each blog post using custom fields. It works for pages too if you add the code to a page template in your child theme.

https://ravenwilde.net/how-to-specify-a-custom-background-color-for-each-post-in-twenty-thirteen/