You need to have Sketch 60+ version and Mockup Plugin.

Download and install a free plugin.

Paste your design

Find 'Paste here" artboard and insert your design into it.

Mockup plugin

First of all, you need to turn on the plugin. To do so, open the Plugins → Mockup Plugin → Run Mockup or simply press ⌃⇧M.

Update

Find the “Change This” layer in the Layers’ panel, and in plugin window you will a list of all artboards. Choose an artboard that you have changed and click apply.

Changeable Colors

You can change the color of each element in the “Change Color” or "Clay" artboards. Find and choose the “Change Color” layer, and you’ll be able to choose any color or gradient in the Fills panel.