Whenever you cast a spell that's both white and black, Nightsky Mimic has base power and toughness 4/4 until end of turn and gains flying until end of turn.
8/1/2008 The ability triggers whenever you cast a spell that’s both of its listed colors. It doesn’t matter whether that spell also happens to be any other colors.
8/1/2008 If you cast a spell that’s the two appropriate colors for the second time in a turn, the ability triggers again. The Mimic will once again become the power and toughness stated in its ability, which could overwrite power- and toughness-setting effects that have been applied to it in the meantime.
8/1/2008 Any other abilities the Mimic may have gained are not affected.
10/1/2009 The effect from the ability overwrites other effects that set power and/or toughness if and only if those effects existed before the ability resolved. It will not overwrite effects that modify power or toughness (whether from a static ability, counters, or a resolved spell or ability), nor will it overwrite effects that set power and toughness which come into existence after the ability resolves. Effects that switch the creature’s power and toughness are always applied after any other power or toughness changing effects, including this one, regardless of the order in which they are created.