Color is automatically applied throughout the deck by selecting a Palette. To change colors in your presentation, go to the Palette button, in the upper part of the Design editor. A list of palettes will show up to select from. 

Each Palette has different Color Sets to alternate colors and give variation.

You can change the color combination on a slide, by applying a different Color Set from the current Palette. For that, go to Background in the upper settings of the editor. Color Sets are the first option in the editing panel that opens up.

To change text color, you can select the text and look for the color button in the formatting menu. The colors available are the ones on your current Palette.

To hand-pick colors for your deck, you can create a Custom Palette. Go to Palette in the upper menu of the editor and look for the "+ New Palette" button, up in the list that comes up. A pop-up will show up with the palette editor.

You can create up to three different Color Sets for the palette. Each set can have three different colors, assigned for texts, backgrounds, and graphic highlights. To select a color, just click on it to access the color wheel or type in its HEX code for more precision.

You can also choose more colors specifically to apply to graphics. Save the Palette to apply it and be able to use it in other presentations. To edit your palette, hit the pencil button visible when you select it.