Yep, mustard works (actually anything that keeps paint out of places you don't want it works), but if you want to go inexpensive but easy to remove, I would suggest rubber cement or liquid latex that can be rubbed off with your fingers, or on the higher end, a masking fluid specifically designed for the task...
Heck, if you want you could you clay or Silly Putty, all you need is to cover the part you don't want to overpaint...
A good option is to use the Search feature with the keyword "Weathered", "Weathering", "Layered" or the like...