(to add some more options to Bpb's post)
Depends on the look you want, see here for some suggested combos.
https://www.atinorthamerica.com/faq.php You will have plenty of power with an 8 bulb over an 18X18 so personally I would swap 2 of the blue for actinic bulbs to put on dawn and dusk channel. I think blue plus alone is ugly but that is personal opinion.