We use a flat surface cleaner and don’t run the washer on too higher reves or it will rip up the tarmac , personally I think Tarmac is the hardest surface to clean and not get stripes we always leave a big overlap on passes to help avoid them trouble is you only see them when it’s dried :
What I am about to say will cause controversy I’ve no doubt we have used hypo on Tarmac to kill moss and lichen for over 10 years never had a problem with bleaching it but we use 15% strength and put 4 parts water to 1 part hypo , another way would be to spray it with weed killer a few days before then wash it if you are worried about bleaching