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its only available via platinum, diamond, and pearl.
you trade the pichu into HG and SS to unlock notched eared pichu in the ilex forest.
although, i think the event ended already, but you can trade for one in the trading thread :D
I think everyone has pretty much summed it up well.
For any further questions, please do use the HG/SS Help Thread.
But yes, the Pikachu-Colored Pichu Event was for DPPt like everyone mentioned.
When you purchase HG/SS, you'll then trade it over to activate the Spiky-Eared Pichu side quest in Ilex Forest.