The three chosen colors must be different from one another.

If you draft another Regicide, any additional colors chosen will count for each Regicide you've drafted.

The target creature can be colors that weren't chosen as long as it's also one or more colors that were. For example, if the chosen colors were red, white, and green, Regicide could target a creature that's white and black.

Regicide can't target colorless creatures.

If a card would allow you to cast a Regicide but you didn't draft one, you won't be able to cast it because you won't be able to choose a legal target.

If you copy or gain control of a Regicide that's already on the stack but you didn't draft one, you won't be able to choose a new target and Regicide won't resolve as it tries to resolve. The target creature won't be destroyed. On the other hand, if you did draft a Regicide, use the set of colors chosen as you drafted cards named Regicide when choosing a new target (if applicable) or determining if the resolving Regicide's target is legal.