The reason it's called being caught red handed is because the expression or saying caught red handed is referring to someone being caught with blood on their hands from either murdering someone or poaching.
Blood is red so you would have red hands as a result of the blood from the murdering or poaching.
Being caught red handed means that you were caught in the act of doing something.
Or if you stole something and were caught stealing the goods and had the goods in your hand then you would be caught red handed.
For example if you were stealing a car and the police rolled up behind you and witnessed you stealing the car or trying to break into the car then you would have been caught red handed by the police.
Or if you try stealing something from a store and someone catches you stealing the merchandise then you would also be caught red handed.
Being caught red handed simply means that someone witnessed you stealing or committing a crime.
Another example would be if you murdered someone and someone caught you murdering that person and you also had blood on your hand.
That could be one of the reasons it's called caught red handed because blood is red and if you murdered someone and got blood on your hand during the murder of the person then you would have red hands due to the blood.
Another example of being caught red handed if you were a child and had your hand in a cookie jar and your mother caught you sneaking a cookie from the cookie jar then she would catch you red handed.
The expression “caught red handed” has its origins in Scotland around the 15th century.
Given how it was used in the earliest references, the phrase “red hand” or “redhand” probably referred to people caught with blood on their hands from murder or poaching.