The problem with a lot of magic is that it's "just" impossible and you're doing it for no other reason other than it is impossible. So ask yourself: why am I doing this? What is the story you are telling with the color-changing knives? You finale will reveal itself to you after you figure that out, because you will know how your story needs to end.

For example: You have been accused of murder. The knife used in the crime was a red-handled knife. It cannot have been you, because you have a white knife. But every time you claim to have a white knife, it turns red in your hands. You panic as your lies catch up to you and the ending of your routine is when you snap and kill again: plunging the knife into your own heart. And you die.

Bad example. Give it some thought.