A new Modern Horizons 2 card, Void Mirror, says, "Whenever a player casts a spell, if no colored mana was spent to cast it, counter that spell." According to MTG rules, how would this interact/combo with Mycosynth Lattice?
In this case, Void Mirror refers to the colors of mana actually spent to cast the spell in question, even if it was spent as though it were mana of a different type (C.R. 609.4b). The mana is treated "as though it were mana of any color" only for the purposes of Mycosynth Lattice's effect, not for other purposes, such as those of Void Mirror (C.R. 609.4). For example, for purposes of Void Mirror, "no colored mana was spent to cast" a spell that costs WUBRG if—
the only mana spent to pay for that spell was CCCCC, but
that mana was spent as though it were WUBRG to cast that spell due to Mycosynth Lattice.
See also this thread.
EDIT (Aug. 14): Correctness edit.