Color on your slides is assigned through a Color palette of your choice. There's several preset palettes to select, and you can create your custom one.
To get there, on the Design editor click on the toolbox on the right edge of screen. On the displayed options select Colors.
You'll find the available palettes, and the option to Add your own palette. A window will pop up for you to assign the colors.
The Plus button allows you to add a new color set to the palette. You can assign up to three different color sets. Each set can have three different colors, assigned to a specific element on the presentation: texts, backgrounds and highlights.
To change a color just click on it, or type in its HEX code.
Once you have your palette and it's color sets, you can change colors on a specific slide. On the Design editor, go to the Slide Options and choose the color set you want to apply for that slide.
Text color is applied for all texts in the presentation, to change it assign three different text colors on your sets, and choose the color you want on a specific slide.