If a Pokemon has no Weakness to begin with, does Oranguru's "Now You're in My Power" attack give it a Weakness to {C}?
Yes. If a Pokemon has no Weakness printed on the card, Oranguru's attack will give that Pokemon a x2 Weakness of Colorless.
Source: Surging Sparks FAQ; TPCi Rules Team (2024-11-07)

Relevant cards

Note: All applicable cards may not be listed below.
SV8_156
SV8_156