Answering myself, it is actually quite simple when you dismiss the idea of using function in any cell.

I defined a sub that is called every time there is change of selection (=moving around cells) on the current sheet.

As the source and destination columns are known, I used a loop to iterate through all the lines in them.
Instead of "ColorIndex" I used "Color" (that makes the exact copy of color and not the nearest one of 56 Excel colors).

If anyone needs it, here's the code (put under "SheetX" in VBA):

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    For line = 1 To 999
        Range("I" & line).Interior.Color = Range("A" & line).Interior.Color
    Next line
End Sub