Make it easier. Make it a natural word. "Help" is good if you don't create a scene where crying for help is part of it. I use "mercy" for that reason and it keeps us "in the scene" should it be a minor issue, question, or just an adjustment that's needed... but if you have trouble remembering, use something very very easy to remember. For that reason, "red" is good... but whatever you choose, do some fire drills.

I often ask my sub if she remembers her safe word and make her tell it to me mid-scene... or as part of her training.