When viewed in reflected white light against a black background, transparent films show the classical interference colours of thin plates which permit an estimate of their thickness to be made. When of the order of 100 nm (1000 Å) in thickness they appear white (silver film) and when thinner they appear gradually less intensely white, then grey and finally black.
Source:
PAC, 1972, 31, 577. 'Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry' on page 614 (https://doi.org/10.1351/pac197231040577)