Simic Keyrune — ruling 1

Until the ability that turns the Keyrune into a creature resolves, the Keyrune is colorless.

Simic Keyrune — ruling 2

Activating the ability that turns the Keyrune into a creature while it's already a creature will override any effects that set its power and/or toughness to another number, but effects that modify power and/or toughness without directly setting them will still apply.

Simic Keyrune : règles, fonctionnement et interactions (MTG)

Simic Keyrune is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Add or . : This artifact becomes a 2/3 green and blue Crab artifact creature with hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

How it works : Simic Keyrune

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
Until the ability that turns the Keyrune into a creature resolves, the Keyrune is colorless.
No other card is named in this ruling.
#2April 15, 2013· Wizards of the Coast
Activating the ability that turns the Keyrune into a creature while it's already a creature will override any effects that set its power and/or toughness to another number, but effects that modify power and/or toughness without directly setting them will still apply.
No other card is named in this ruling.