What color is this effect?

[Lorn Asbord Schutta]

Blanket
Instant/Sorcery (?)
Blanket deals damage to target creature equal to its toughness. Excess damage is dealt to that creature controler.
"A flavour-text fitting to flavour."
-Wise Man

As above. I am pretty sure the rarity would be uncommon, since you have to know about damage marking to understand this. I could see it mostly in black or red, but white and green could make some claims too.

[Krishnath]

It's red. It is a direct damage spell, and the few direct damage spells we've had with spillover have all been red (I.e. Flame Spill.)

[SecretInfiltrator]

Additionally reasons this doesn't particular fit white or green if they would "make some claims":

It is outside greens "creature removal through creatures"-scope. Green could comfortably deal damage equal to one of its own creatures toughness, but this spell is more Wrack with Madness than Rabid Bite.

For white this would be an uncomfortable bend (no break since white gets straight creature removal at a cost) because the conditions don't feel particularly white. This is strongest against a damaged creature with high toughness, while white is (outside of aggressive weenies) a color associated with high toughness and punishes creatures for being aggressors (dealing damage/attacking/tapping etc.) rather than victims (in this case receiving damage) - even if this can be associated with entering combat as a blocker. I. e. this is Crushing Pain rather than Reciprocate.

On black:

Black could get this, too (unless the "excess damage" part gets arbitrated as not available to black).

On red:

This is available to red only insofar as you accept Shivan Meteor as a red card. Red is supposed to invest an effort into dealing enough damage to overcome the toughness of a creature, so high amounts of damage or those that are defined in such a way that they always reach the lethal threshold (as is done here) are a bend in red for that reason.

I wouldn't bat my eye at this is mono-black, black-red hybrid, or mono-red. Yet I acknowledge that the combination of the two effects makes it not a perfectly neat fit for either black or red. The bend is not all that severe in black, but the means are unusual.

[Mookie]

This reads to me as being at least partially black - 'deal damage to target creature equal to its toughness' is functionally 'destroy target creature'. I which isn't an effect that red has access to. That said, I don't think monoblack could get the damage spillover either - that usually shows up on red cards like Repercussion and Flame Spill. So, I'd put it as a Rakdos gold card, although it may also qualify as a bend in either monored or monoblack.

[void_nothing]

BR seems to be where this would fit, yes.