This is RGBA color. There are many ways to choose color, here you can read about some of them Color Codes: What's the Difference Between Hex, RGB, and HSL? - Make Tech Easier. You can also use colorpickers (for example, this https://colorpicker.me/) to get code for color you want (note that if RGB is presented in range 0-255, you need to divide each channel by 255, because Godot use percent-based values (in range of 0-1)).
Very detailed answer additionally you can use Godot’s built-in Globals Color.blackColor.redColor.blue etc…