While you can even pulse coarsely chopped peppers in the food processor for the Salmon with Tri-Pepper Salsa, to julienne them is to create something really elegant looking. The best way to do that is to “fillet” the peppers.
With a straight cut, slice the top and bottoms off the peppers. Make one vertical slice through the flesh and lay it flat on the cutting board as if it were a a book. With the blade of your knife, slice the white pith and seeds away from the flesh. In the end, you want one very long rectangle that is free of the pith and seeds. Press the long rectangle to the cutting board and slice as thinly as you can the bright colors into skinny, skinny sticks.
Making rainbows on my cutting board