I used to mark the center line of my rough limbs and the profile to be cut with a blue ball point pen(on brown masking tape).
I almost ruined a bow once. I had cut one side of the profile then started to cut the other side but I was on the centerline instead of the profile line. Since then, I mark the center line with red ink and the profile lines with blue ink. I NEVER cut the red line! I have however, almost cut on the wrong side of the profile line! If I cut to the right of one line, my brain wants me to cut to the right on the other line.
Hope this helps someone someday!