Since Celestial Dawn no longer changes the Mana symbols in the casting of a permanent, does it only count as white or would a creature that normally costs all wubrg count as a five color creature?
Celestial Dawn's effects work only on the battlefield, not in any other zone (C.R. 112.6, 611.3b). If you control Celestial Dawn (C.R. 109.5), then in general, among other things--
spells you control are white, including creature spells, regardless of any costs required to cast them (C.R. 613.1, 613.1e, 613.4),
nonland permanents you control (including Celestial Dawn, if applicable) are white, regardless of what colors they normally have (C.R. 613.1, 613.1e, 613.4), and
to cast a spell that costs WUBRG, you can spend white mana and only white mana to pay that cost ("[y]ou may spend other mana only as though it were colorless mana", but colorless mana can't pay for the colored mana portions of a cost [C.R. 107.4a]).
See Celestial Dawn's Oracle text, which no longer speaks of mana symbols (C.R. 108.1).
Since Celestial Dawn no longer changes the Mana symbols in the casting of a permanent, does it only count as white or would a creature that normally costs all wubrg count as a five color creature?
EDIT (Sep. 18): Added rule citation; correction.
EDIT (Mar. 31, 2020): Clarification.