Is it possible that your 500 defaults to PAL (hardware default, a 1 Meg Agnus is switchable) and is then switch to NTSC by a Prefs setting? Unfortunately 1.3 lacks the early startup screen where you could easily switch the hardware.
BTW: A520s are either PAL or NTSC. There's no way to get any colors on an NTSC monitor/TV with a PAL A520 - that rules out any mismatching here.