Scuttling Sentinel

Flash
Vigilance
When this creature enters, put a +1/+1 counter on another target creature you control. Until end of turn, that creature becomes a blue Crab in addition to its other types and gains hexproof. (It can't be the target of spells or abilities your opponents control.)

Ravnica: Clue Edition

Flash
Vigilance
When Scuttling Sentinel enters the battlefield, put a +1/+1 counter on another target creature you control. Until end of turn, that creature becomes a blue Crab in addition to its other types and gains hexproof. (It can't be the target of spells or abilities your opponents control.)

The target creature will not retain its previous colors; it will just be blue until end of turn.

The target creature retains any types, subtypes, or supertypes it has.

The target creature will become a blue Crab in addition to its other types and will gain hexproof even if counters can't be placed on it for some reason.