Background Color dialog box
Use this dialog box to choose a background color for an overlay, or to choose a background color for cover pages in the Book Settings dialog box.
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Click and drag the color slider control until you see a range of desired colors in the large box above the slider control.
Then, click a place in the top part of the dialog box that shows the specific color you want to use.
You can click and drag the mouse pointer to see the colors change in the small boxes.
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For background colors, click and drag the transparency slider control until you see the desired opacity. -
Type or paste a hex color code (https://htmlcolorcodes.com/) in the box.
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Click a recently used color to use it again.
For examples:
or 
Note
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The Text Color dialog box is similar, but not does not consider opacity.