How to put a while true loop in a for loop?

Just to mention it:

Q: Why can't I have a while loop in a for loop?

A: You actually can, but you would need to break the loop at some point for it to to continue with the previous loop. You can still create a new thread (using coroutines or spawn) which will run at the same time. However, if you do this, after a couple of times, the performance will decrease - and this is generally bad practice.

Now, to the solution, what you would do instead here is to create a table where you put the parts in after you create new parts, and then create a separate thread that will change the color of the parts in it after X seconds.

Here is a simple solution:

-- // Variables
--------

local changeColorParts = {};
local changeColorDelay = 0.1;


-- // Creating a single part and inserting it to the table
--------

local newPart = Instance.new("Part");
newPart.Parent = workspace;
table.insert(changeColorParts, newPart);


-- // Creating a new thread and running it
--------

coroutine.resume(coroutine.create( function ()	
    while true do
	    -- Looping through the parts in the table
		for index, part in pairs (changeColorParts) do
			-- Changing its color to something random
			part.BrickColor = BrickColor.Random();
		end
		-- Waiting .1 seconds
		wait(changeColorDelay);
	end
end)

This is pretty simple, it basically has a table called changeColorParts and all parts inserted to the table will have their colors changed every changeColorDelay seconds, which in this case is every 0.1 seconds.