Color in your slides is assigned through a Color Palette. You will find a list of Palettes in the Design editor, each one with different combinations. These combinations, or Color Sets, are determined by professional design considerations in order to ensure a good result and make things easier for you.
You'll find the "Palette" button on the top side of the Design editor.
Just click on it to get the list of available options. The colors will be automatically applied and sorted through all slides and changing the palette will affect all the slides. Each slide will have a different combination to ensure variety and you can change this for each individual slide, within the Color Sets in the selected palette.
In order to change color in a slide, try the different Color Sets. Find this by clicking on the Settings button, on the right side of the slide. This change will apply only the current slide.
Creating a Custom Palette
If you're on a Premium or Professional plan, you'll be able to create your own Custom Palette. Within the Palette options, use the "+New" button to create your palette and be able to add up to 3 different sets.
Each set can have three different colors, assigned to a specific content block of the presentation: texts, backgrounds, and highlights. To change a color in the palette, just click on it, or type in its HEX code.
Changing Text color in the slide
Once you have your palette and it's color sets, you can change colors on a specific slide. In the Design editor, you'll find the "Settings" button at the side of the slide. Click on it and change the Color Set available in the options.