TO BE CAUGHT RED-HANDED

There is a murder, who could the murderer be? In Spanish threre is a similar expresión who answer this questions: "EL QUE TENGA LAS MANOS ROJAS". The translation means: The one whose hands are red. That expresion is for children because if they are guilty they would look their hands to know if they are red or not, or they would try to hide them.
Maybe, red colour could have the blood connotation, thats the reason of the chosen photograph.