I looked for quite a while to find reasonably priced paint pens. I found these paint pens on EBay to label my plastic pots. They are as good as the expensive ones. It took over a month to get them, but they work great and are good quality. The paint will not rub off the pots that I just tried. Only 99 cents each. I ordered 5 white ones. They are a fine/medium tip, (not a wide tip) but any wider would almost be a waste of paint. They are made of aluminum and have a little mixer/shaker ball inside. Good value.  Go to ebay and enter this title that I copied and pasted below. Hint: Might work to label a tree itself if it was needed.   

Colorful Tyre Permanent Paint Pen Tire Metal Glass Lettering Marking Ink Marker

Its hard to find a good paint pen. Thanks.

Hello,
  It is best to grow figs trees in light colored pots. Black pots (containers) will become very hot during hot summer months which will damage roots and lessen fig or fruit production. This is my experience with dark pots. 
Phil from Northern Ohio.

I haven't experienced overheating of black pots. Nurseries grow Millions  upon millions of tree's in them. I've grown different fruit tree's in them and never had a single issue. I'm in Zone 8b and we do get some HOT weather 'but I do water almost daily'.  Maybe a lighter color would cut down on the watering. I usually cluster my pots though so the ones inside the  group probably wouldn't be affected anyway. I will have to experiment and see. Either way, the paint pens are great and you could buy whatever color would work best.