A color palette is a selection of complimentary colors associated with various elements of the UI program, such as background color, headlines, body copy, icons and buttons, etc. Usually 4 - 8 colors are selected from the web-safe 216 palette. Clarity and legibility are the main objectives, but contributing to the "mood" of the application is also important. A good resource to develop a color palette is https://color.adobe.com/
You can use the "Extract Theme" feature to create a palette based on a photo, such as a picture of your client's product(s) or their storefront. Or you can create a custom palette of your own using the color wheel. If you create an account you will be able to save your palettes to an online library, or you can just download them as a jpeg file (or CSS).
When you have created your color palette, download it as a jpeg file and add it to your "Design Brief" page on your project web site (below the Persona section). Email the URL to uidev@cs.stonybrook.edu with "Palette" in the subject line.