Thoughtlace 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

Target spell or permanent becomes blue. (Mana symbols on that permanent remain unchanged.)

Complete guide

Thoughtlace is a classic utility instant from the early days of Magic that allows you to manipulate the very essence of a card. By shifting the color of a spell or permanent to blue, you can bypass protections or set up powerful elemental synergies. While it doesn't remove original abilities, its color-changing effect lasts indefinitely for permanents on the battlefield.

What It Does

Thoughtlace targets a single spell on the stack or a permanent already on the battlefield and changes its color to blue. It is important to note that this is not an additive effect; the target stops being its original color(s) and becomes blue only. The oracle text clarifies that mana symbols on the card do not change, meaning a Red spell changed to Blue still requires Red mana to cast, even though the spell itself is now Blue.

How It Resolves Step by Step

First, you cast Thoughtlace and choose a legal target, which can be any spell or any permanent. Upon resolution, the target's color attribute is overwritten. If the target was a spell, it remains blue as it resolves and enters the battlefield as a blue permanent. If the target was already a permanent, it becomes blue immediately and stays that way as long as it remains on the battlefield.

Common Mistakes

A frequent misconception is that Thoughtlace changes the mana cost or the color identity of a card for Commander deck-building purposes; it does neither. Additionally, if you use Thoughtlace on a spell, and that spell is countered or otherwise leaves the stack before Thoughtlace resolves, Thoughtlace will fizzle due to an illegal target.

When to Play It

Thoughtlace is most effective when you need to interact with 'protection from' effects or cards that specifically target colors. By turning an opponent's threat blue, you might enable your own cards that have bonuses against blue permanents, or you might save one of your own permanents from a spell that targets a specific non-blue color.

Interaction examples

Color Shifting on the Stack

Counterspell

If an opponent casts a spell, you can cast Thoughtlace targeting that spell. Once Thoughtlace resolves, the spell is now blue. While this doesn't stop Counterspell from working (since Counterspell can target a spell of any color), it could be relevant if you were using a different countermeasure that only targets blue spells.

Interacting with Noncreature Spells

An Offer You Can't Refuse

You can use Thoughtlace to change the color of a noncreature spell to blue. After Thoughtlace resolves, you can then target that now-blue spell with An Offer You Can't Refuse. The color change is independent of the spell type, so the spell remains a noncreature spell and a valid target for the counter-magic, regardless of its new blue hue.

Guide written with AI from the official text and rulings. Official English text prevails.