Anyone who has tried to label all of their plants will know that there are many ways it can be done. Probably most common and convenient are the white plastic labels and the black marker pen. Other methods include wax/indelible pencils, black labels and white pens, scratched coated labels, embossed aluminium plates of various designs and imprinted copper. I have also used embossed adhesive based plastic labels as produced by a machine. Many of these are expensive when you have significant quantities to produce. The nurseries use pre-printed labels, but these are not practicable for most home users.
Eventually the 'permanent' pen marking on plastic labels always fade in sunlight. And well-marked labels might need to be re-used. I have found that old labels can be cleaned with methylated spirits, and then they can be re-used. There is of course a cost trade-off, but if you collect together a number of labels and soak them briefly in a small shallow dish of meths, they can be rubbed sufficiently clean to be re-used. It will also de-grease the surface. If you try this, be very careful because meths is highly flammable and volatile.
One of our members tells me that she uses the white plastic ends from date boxes to provide a nice white plastic substrate on which to write. Cutting them to size and using a narrow tipped marker works well. Wooden lolly sticks also work well. and have the advantage of being bio-degradable; however, it also means that they don't last long!
For those of you who would like to investigate further, look at The Essentials Company. They offer a wide range of methods.