The colors are found in a folder named "Colors" found within the main script.
Locating the Colors folder
Adding Colors
To add a new color, click the + and type "Color3Value" if it is a solid colour or "UIGradient" if you'd like multiple colours (or just duplicate an existing color).
Solid Colors
Gradient
Rename the newly-created value to the name of a unique color (a color that isn't already in the list).
Next, find the "Value" (Color3Value) or "Color" (UIGradient) property. Click on the property and you'll see 3 dots or a color wheel.
How to edit the color for Color3Value
How to edit the color for UIGradient
After creating a color, add the color name to your tag or nametag customisation!