Each time a Spell Card is activated, place 1 Spell Counter on this card when that Spell Card resolves (max. 3). You can Tribute this card with 3 Spell Counters on it; Special Summon 1 "Dark Magician" from your hand, Deck, or Graveyard.
If you summon Skilled Dark before you use a field spell, you can use the field spell card, murmur, and then re-use the field spell to get 3 counters quickly in a turn. Spell Power grasp can add 2 spell counters with 1 card. You can use cards like Dark World Dealings for a quick and always usable at all times kind of card to get a counter. Cursed Bamboo can also be a good way to get 3 spell counters quickly, but you will need other bamboo equip cards.
I wonder why this card cannot do what Red, White and Blue can do from the graveyard. Red, White and Blue all say you can banish from graveyard to add 1 spell counter to a card. But Skilled Dark Magician does not say that.
I'm just wondering why...
Because this was the original "Skilled ... Magician" card. White is also very old and doesn't have the effect you've mentioned. The Blue and the Red ones are quite new cards though, that's why they have better effects.