Red
It won't come off.
The towel someone hands her in the room stains crimson.
The bathroom in the building is ruby-splashed.
The sink at the precinct bears no sign, but she slips away to the evidence room and finds the UV light and her fingers still glow.
She watched her die.
Kate Beckett took that scalpel out of a murderer's hand and pinned her to the ground, found one carotid, sliced lengthwise, and as she screamed, found the other and did the same.
Then stood and watched.
Gunshots are clean. They make nasty scars, but a shot to the heart, or the base of the brain? Neat. Tidy. Immediate.
Scalpels are exact instruments but inefficient weapons.
Kelly Nieman sputtered, she screamed, her blood spurted with the pressure of a healthy, beating heart.
The surgeon's face went pale as the color from her cheeks sprayed over Kate's skin.
And now it won't come off.
Kate Beckett stands in her shower, scrubbing, smelling blood, seeing green grass and slick white linoleum, both flooding red, feeling the air drain from her chest, trickle down through the grate.
Castle finds her, cold on the floor, eyes wide and blank, stiff, just how she left a murderer who never did take a life.
The red is gone.