You can add a color palette strip to your board by clicking on the icon present at the bottom of the board as shown:

Clicking on  "+ Add Color Strip" will direct you to two choices:
1. Create New
2. Open Saved

1. As the name suggests, "Create New" will allow you to create a new palette by clicking and dragging color swatches from the left side of the webpage - under "colors". To accommodate every need possible, we have added various color guides for your conveniences - Pantone, CSI, and Archroma color atlas. You have multiple options for finding and picking colors for your palette. 

Each of these options is explained in detail here

2.  The "Open Saved" option will let you access saved and shared palette/s and open as a color strip. Clicking on "Open Saved" will open another window -