I have found a "Edit master" tool in "Slides" menu, but even though I change background color of certain elements, this color does not appear in the "template colors" menu when I want to pick a color for something else. For example, if I choose a "recolor" option for some visual elements, I can only choose from theme colors, but I can not figure out how to edit them.
Figured it out! I was having the same problem.
Click on any slide--then at the top click 'background'
Click on the drop box across from 'color'
Then, click on the little pen icon beside 'theme'
Click on the 'choose a theme color' drop down box
That should take you to a menu where you can access all the theme colors and edit them.
The answer above didn't work for me because I couldn't find a "Palette" option anywhere. The only thing that worked for me was creating a theme in PowerPoint with a custom color palette, and importing the theme into Google Slides. (Yeah.) I opened a new blank Powerpoint deck, went into the "Format" menu, to "Theme Colors," changed the colors to match my desired theme in Google Slides, saved the customized color palette, then selected "File" > "Save as Template." I went into Google Slides, imported that template and that changed the Theme Colors in my Google Slides document. BEWARE!!! The theme you create in Powerpoint will overwrite any master slide/layout edit's you've already made in Google, so you may want to do this before getting too far in Google Slides, or create a PowerPoint template that matches the layouts you want in Google. (I had to go in and change the Google master fonts after importing the PowerPoint, because Google uses web fonts.) If you have a PowerPoint version of the deck template that you're building the Google Slides from, just import that and you should be all set.
Updates This official feed from the G Suite team provides essential information about new features and improvements for G Suite customers. Easily customize theme colors in Slides April 15, 2019 Quick launch summary You can now edit the theme colors for your presentations in Slides, making it easier to set up a new theme, or apply bulk color changes to existing presentations. Previously, we only supported custom colors that were not linked to the presentation theme. Now, users can easily customize the color palette associated with a theme. To do this, go to any color picker dropdown and click the edit button for your Theme color palette. In the Theme color sidebar, select a color from the dropdown to begin editing.
Note: These changes will only apply to the current theme and will not create a new theme. We hope this new feature will help you customize your presentations in Slides, especially when it comes to incorporating your company’s brand colors. Availability Rollout details Rapid Release domains: Gradual rollout (up to 15 days for feature visibility) starting on April 15, 2019. Scheduled Release domains: Gradual rollout (up to 15 days for feature visibility) starting on April 15, 2019. G Suite editions Available to all G Suite editions. On/off by default? This feature will be ON by default.
Wherever you get to choose a color in the Slides editor you are presented with a color picker tool. As part of this tool you can choose from a preset color palette. This palette can optionally be customized with your own set of colors based on the active theme which is a great way to help team members stick to your intended color scheme. To define a custom palette start by opening the theme editor and clicking the "Palette" tab in the top right. Once there you can pick the color you want and press the "Save color" button to store it. The tiles in the bottom of the page represent colors that you have saved. They can be reordered via drag and drop.