silver film

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