Previously presented by Microsoft as "ThemeSlots", the SharePoint Color Palette Tool will help you create your own .spcolor file (Theme) for a SharePoint 2013 Composed Look.
The interface presents you with fields to input your color values. Preview your choices by using either the Seattle or Oslo layouts as well as handy Contrast Test and UI Preview layouts. You can also bring in your background image to assist in your testing.
This is much easier than manually editing a Theme file in Notepad or SharePoint Designer because the color value fields can be grouped to assist you--by UI Groups, UI Type, and Color Buckets. The Color Buckets grouping is particularly helpful if you would like to tweak an existing theme's colors because all intentionally similar attributes get grouped together. You can then change just a single field and maintain the color value consistency of the other attributes that should match across the theme.
Don't know where to start? Pick a single color using the picker to start from and then click the Recolor button. The tool will automatically suggest variations for all of the other attributes.
Any errors and/or warnings across the Theme are presented to you as you go.
You will need to upload your .spcolor file to the _catalogs/theme/15/ folder of your site collection in order to select it in Site Settings.
P.S. - Keep in mind that SharePoint 2013 supports RGBA color values, not just hex! The 'A' is for the alpha channel, allowing you to control the opacity.
Related Links
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"Color Palettes and Fonts in SharePoint 2013" by Microsoft
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