I explore lots of colour palettes when creating prototypes. I have found this Sketch plugin by Alessandro to very helpful for this. It allows you to import different colour schemes from Colourlovers directly from Sketch. It also has lots of other features to aid aligning shapes.
Colorful Gradients

Will Burn released an awesome website that programmatically generates different colorful gradients and color in color images. I used some of these to spark ideas for variations of designs. To automate this into my workflow I created a Sketch plugin to import these background from within the app.
Color in Color

Usage

Select Shapes +
Plugins > Menu > sketch-colorfulgradients > OptionWait for the image to be downloaded and inserted as a fill
Installation
Official documentation is here.
Clone
Find Sketch Plugins folder using
Plugins menu > Reveal Plugins FolderE.g.
git clone /Users/dunc/Library/Containers/com.bohemiancoding.sketch3/Data/Library/Application\ Support/com.bohemiancoding.sketch3/Plugins
Download
Find Sketch Plugins folder using
Plugins menu > Reveal Plugins FolderDownload here
- Unzip and put into Sketch plugins folder