Well I'm not going to give you the exact code because that would be what we call "spoonfeeding" but I will show you the concepts you need to understand in order to create it
We will start with the "sign", i assume the sign has a property where it displays color
How to change properties
Now about the "when a specific person joins" there are two parts to this, firstly, we need to detect when a person joins, this is called an event. An event allows you to make certain code happen whenever the event occurs
Events
The certain event when a player joins is called PlayerAdded
And finally, the "specific person" part, so far, if you learned events and playeradded and all that, you now have the player. The player has a property, which is it's Name. To check if it's name is a certain name, we would use an if statement to check if the name (which is a string) matches the name of the specific person
Alternatively, you can also check if the person's UserId is equal to another UserId just in case their name is changed, and UserId is a property of player and is a number that you can find on your profile page url