In JS this is used almost as a fallback. It’s not that it returns one or other, but it will return the one that is not null or undefined.
In GDScript u can’t do the same. What u can do in this case is use a ternary approach like:
var current_color := user_color if user_color != null else default_color
Edit: Btw i didnt saw @trizZzle comment 
. It’s pretty much the same, sorry.