Multiple comboboxs making unique selections from same data

Hi,
I have been trying to get a set of userform comboboxs pulling from the same (or similar) data source to only show selections that have not already been chosen, independent of the order that the comboboxs are selected.
e.g. In the case of the common data being "Red", "Green", "Yellow", "Blue".
If I select "Green" with box1 then it is not available in the lists for boxs 2-4.

I have tried searching a few forums but have not found a solution yet.
I am not fussed how comboboxs get their initial data (I first used selecting the row source)

The below example code works only in a certain order.
I think the errors occur because when I remove an item from the listindex it reorganises the selection so if I choose index 0 item and it is removed from the list then on the next combobox item 1 now becomes item 0 and if that is selected it has a with the fist combobox already having selected that even if it is a different value.

Any help appreciated.

Formula:

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Thanks for the help dflak.
I had a look and that and tried to get my head around how it worked
One thing I thought I saw and confirmed was that since a combo box with a already selected value is not cleared, then as you add items to the boxes you get double ups being applied to those boxes if you were to go back to them.
By nesting the combobox value check when redefining the tables this counters that issue.
But then the second selection can still be selected by the first combo box if you go back to it.

Are there any other ways that one list of data can be used for multiple combo boxes with no double ups of chosen data?