How can I change the properties of multiple parts?

function whenClicked()
	local plr = game.Players.LocalPlayer
	local playerpos = plr.Character.Torso.Position
	local rightlightpos = game.Workspace.FlashLight2.Union.Position
	local difference = playerpos - rightlightpos
	local number = 50
	if difference.magnitude < number then
		local xa = game.Workspace.Lobbthing.Part
		xa.BrickColor = BrickColor.new("Black")
		
	
	end
end

script.Parent.MouseButton1Down:connect(whenClicked)

The purpose of this script is to change the color value of a part within a model called "Lobbthing", once a player is a certain distance away from another object. However, my goal is to make this script work for all of the objects in the model called "Part" and that does not happen. I know that there is a way to do this but at my current stage within this language I am unable to do that. Note: This script activates when two conditions are met:

1. A Button in a GUI has been clicked.
2. The Player is less than 50 studs away from another object, in this case being rightlightpos.