Style text in Numbers formula
Although one cannot style individual characters in a formula, one feature of Apple Numbers is the canvas.  One can take advantage of this by adding tables and text objects anywhere.



The above screen shot uses a static (as the OP's "Invoice terms" text is hard coded in their formula example, it is assumed static) text field placed next to the table (a single cell table with no border, in this case) containing the cell with the formula.

The OP's use of the CONCATENATE function is unneeded as the ampersands do the same job.  To use CONCATENATE in this case:

=CONCATENATE(Terms,", please pay by...")


Where Terms is the reference to the cell containing the Terms text.

Detailed screenshot:



By using a number for the days, this can be used in a calculation against the Invoice date to determine the date payment is due.  But that is a topic for another question. ;)