Basically an ArrayList is not an array, it is just a collection of objects.
With an array, if I create a 5 length array then I get a structure that can
hold 5 elements of my given type.  So I can then do;

arr[0] = "Red"
arr[1] = "Blue"

etc.

When you create an ArrayList, you merely get an object that is an empty
collection, it has no objects in it so you can't do

arr[0] =

as there is nothing at position 0.  For there to be something at position 0
you have to add it;

arr.Add("Red") -- this is the first thing added so will have position 0

You're just getting confused between arrays and the ArrayList object,
they're not the same.  This syntax;

arrColors(0).Add("Red")

is the same as this

Dim o as Object
o = arrColours(0)
o.Add("Red")

What you're doing is getting the object at position 0 then calling that
object's Add method.  As well as the problem above of there being no objects
in the array, if there *was* an object such as a string "Red" then you're
calling the Add method of that object which can give you errors if the
object doesn't support it.  You are *not* calling the Add method of the
ArrayList, you are calling the Add method of the object at that position.

Again not really as they are just collections.  The ArrayList will grow and
shrink in size as you add/remove elements.