Highlighters. Even with color coding and bolding, you're likely going to need another level of emphasis. Maybe to highlight (literally) plot twists or foreshadowing, themes or critical character development. Highlighters are the classic best tool for the job. I don't care much for the typical marker-style chisel-tip highlighters. They do work, but too often I dislike the way they bleed through paper or smear ink. Neon colored pencils are good, but aren't as quick, since it takes some scribbling to cover an area, instead of one swoop. My favorite tool for the job are the Staedtler Textsurfer Gel highlighters, which are basically super-soft gel crayons. You can highlight in a smooth swoop, and the soft wax doesn't bleed through paper or smear ink. One downside, though, as the wax makes it impossible to write over that area again at all.
And of course, the non-stationery ingredient to this list is flexibility. Outlines are the best example of the "guidelines not rules" adage. You're in charge of your story and you can change your mind anytime. Make your story work for you. And if your outline isn't fitting, change the outline! I have three outlines for my current novel, where the story trajectory changed and I needed to re-route things. Maybe add some scissors and scotch tape to this list, too. And a glass of wine.
Tell me your favorite tools for planning your writing projects! Everyone's method is a little different, and I find them all fascinating.
(JetPens provided these products at no charge to The Pen Addict for review purposes.)