Indelible as in UV-proof, or sunlight will fade the ink. Only pigment-based inks will survive a few years of sunlight. Dye based inks might manage to last a year.
Pigments are very finely ground minerals, while dyes are organic chemicals. Paint uses pigments, while many inks use only dyes.
India ink works, as does pencil. The pigment is carbon black. Some kinds of india ink are designed for plastic surfaces.
Sakura Color in Japan may make markers with suitable inks. I haven't tried them yet, but the writeups on some of their products sound promising. These are widely distributed in art stores in the US.
Sharpies (Sanford) fade quickly, even though these are very good markers.
Joe Gwinn