You can buy a white toner cartridge to use in a standard laser printer on clear decal film. You just make your design in black, then swap out the black cartridge for the white one before you press ‘print’. Here’s an example.
The same thing used to be available for ink-jet printers (maybe 15 / 20 years ago?), when I was making decals for model helicopters, and the results were ‘OK’.
Now (I believe) some hobbyists use the Epson Eco-Tank and fill one of the tanks with white textile ink.