which quarks have a positive charge
Why do quarks only have charges of +2/3 or -1/3? Not 1/3 or -2/3? An up quark (+2/3) would have a positive (+) charge type mono-charge with a +1 charge strength join a negative charge type mono-charge with a -1/3 charge strength to get a net +2/3 charge for that formed elemental particle. Conversely, a down quark would have a -2/3 mono-charge joined with a +1/3 mono-charge for a net -1/3 charge.