Aside from documentary or photographic evidence, neither of which was available for the Colonnade columns, the best way to date paint layers found on an architectural element is by comparing it to a similar element from the same house that has a longer paint history.  Longer paint histories provide additional opportunities for analyzing the composition of the different paint layers (which can provide clues as to their relative dates) and can be more accurately aligned with any major changes made to a building.  In this case, the closest element with a surviving paint history proved to be the wooden colonnade pilasters.  Because the finish for the pilasters would have mimicked the column finishes, except for the plaster render found over the bricks, the paint history for the pilasters would make a very good comparison for the surviving paints on the columns.