The Ancient Green Dragon can inflict a target with ongoing 15 poison damage (save ends) with its Bite attack (as well as immediate normal damage). It also has an ability it can use on its next turn called Mind Poison which "only affects a target taking ongoing poison damage" and inflicts -2 penalties to checks, dazed, and finally a form of charm/domination.
Similarly the Zythar can inflict fire damage and ongoing 10 fire damage on a target with Flame Bolt, and has another attack called Tendrils of Flame which can only target "one creature taking ongoing fire damage", which does extra damage and also strips the target of its fire resistance (save ends).
If a character has resist 20 poison and resist 20 fire and is hit with the initial attacks which cause damage and ongoing damage, but suffers no damage from the ongoing damage due to resistance, can it still be targeted/affected by the follow-up attacks? Technically the character would still be coated in poison or would still be on fire, it would just be suffering no damage from them, so abilities which manipulate the poison/fire on a target might still apply. Would you still roll to save against non-damaging effects in the first place?