Color changing on medical tools

Internal use plastic medical equipments marking must be permanent and smooth. No material leakage or friction is allowed inside the human body. Moreover, small size is an important requirement too.
Urinary catheters represent such a case. Customer’s request is to mark a readable scale, made of ticks and numbers, on these 3 mm diameter pellethane tubes. Material color is white, so dark contrast is what we are looking for, without any surface foaming or burning.
A laser marker, with its 355 nm wavelength, is able to change the surface color from white to grey.