How do you update the formatting of a cell using xlsxwriter?

I am generating an excel file from Python using XlsxWriter.

I'm trying to "layer" formatting depending on the cell value. In pseudocode:

for rowId in rows:
    for colId in cols:
       value = table.loc[rowId, colId]

       if value < 0:
           # Make font red

for rowId in rows:
    for colId in cols:
       value = table.loc[rowId, colId]

       if value > 1e6:
           # Make bg orange

for rowId in rows:
    for colId in cols:
       value = table.loc[rowId, colId]

       if value is False:
           # Make bg purple

In practice I'm finding it hard to layer formatting without either creating a format object for each combination of attributes, or creating a format object per cell.

Any ideas of how this could be achieved?