Evolving Door's ability instructs you to count the number of colors, but it doesn't track what those colors were. For example, if you sacrifice a creature that is green and no other colors, you'll search for any creature card that is exactly two colors, even if the card you search for isn't green.

If you sacrifice a colorless creature, you must search your library for a creature card with exactly one color. If you sacrifice a creature that is all five colors, you won't be able to find any creatures because there are no creature cards in Magic with six colors.

You must pay all costs for a spell you cast this way.

If you choose to cast the spell, you must cast it as Evolving Door's ability resolves. You cannot wait and cast it later.

You count the colors of the sacrificed creature as it last existed on the battlefield. Notably, if it was a double-faced card or a copy of another creature, that may be different from what its colors are in the graveyard.