Pigments with particle size less than 0. 1 micron (i.e., less than 100 nm) are transparent and colorless because their particle size is smaller than the wavelength of visible light (320-750 nm).
There are many kinds of nano-scale pigments, including organic/inorganic pigments and transparent iron oxide pigments.
Nano-pigment manufacturing technology has been very common, but its core technology is filtration and separation technology. How to strictly control the particle size below 100 nm? At present, most of the best ones use German patents.
Fundamentals of Nanotechnology and Production Technology can be bought in bookstores and published by Chemical Industry Press. Space is limited here, so I can't describe it in detail.