I can apply and touch up cerekote when I need it, BUT nitride probably won't ever need it.
If you have a parts store, try a can of ceramic caliper paint. Heat your metal to 120-150 degrees, mist on a light base, let dry, put on another coat, let cool, and top it in whatever color you like. Finish with dry silicon spray as it cures. That's not coming off with regular use unless you use zip strip vandalism spray.