I'm still baffled by this. I know the whole "multiple replacement effect of the same event, the owner chooses which one happens" stuff, but the necro replaces the discard event, whilst the colossus replaces the "put into a graveyard" event. This is hardly the same event is it not? When you have 9 cards, you have to discard, which is replaced by necro. Colossus doesn't replace the discard now does he? His replacement effect only happens if it would go to the graveyard. But it never does because necro happens before that can happen IMHO.
Actually, that's a good point and a decent argument. By your line of thinking, the [card]necromancy[/card] makes you take a card from your hand and RFG it before it would ever go to the graveyard so the even the [card]dark steel colossus[/card] replaces never happens. It makes some sense.
This is the closest thing I could find to refute it and I believe it is the basis of the 'same event' argument. The definition of 'discard' is:
* G4.12a - A player discards a card by
putting a card from his or her hand into his or her graveyard. By default, effects that cause a player to discard a card allow the affected player to choose which card to discard. Some effects, however, require a random discard or allow another player to choose which card is discarded. [CompRules 2003/10/01]
The event, taken as a whole single event (which is what 1 discard is) involves placing the card in the graveyard. Discarding a card IS placing it in the graveyard, which IS what DSC replaces.