Idle question: Color of tape coatings

The following answer comes from a retired IBM tape technologist: The color of the coatings on a tape are dominated by the magnetic pigment (or the carbon used in back coats..which is black)..the earliest iron oxide coatings were based on the conversion of alpha-iron oxide which is a pale yellow and some what needle shaped (longer than wide) but non-magnetic to gamma iron oxide which is magnetic. The very early particles were not very homogeneous and were very highly aggregated....poor conversion as the early particle were being produced by paint pigment manufactures as a specialty product in low volume. These were a yellow brown color..but by the 60's gamma iron oxide of 250-300 Oe was commonly used in the magnetic layer coatings. These were typically brown to chocolate brown (if they had some carbon black mixed in for surface conductivity (anti-static) which depending on the use as well as the manufacturer varied a lot i n surface finish (gloss) as well). This market was driven by audio primarily and dynamic range and analogue signal characteristics such wow & flutter were driving formulation and magnetic particle development. In the late 60-s and 70's new particles began to enter the market..Cobalt doped and later cobalt 'modified" gamma iron oxide as well as chromium dioxide..and some very early explorations of iron metal particles and some exotic mixed metal crystals... The colors of the magnetic coatings based on more acicular gamma iron oxide made specifically for the recording market were now reddish brown , cobalt doped were a dark brown - to black, chromium dioxide is very black..remember during this period digital recording in both tapes and disks were now the growth areas driving new pigment development and drastically improved formulations driven by the need for improved durability, longevity and wear characteristics (drop outs (defects & debris), head wear and head/drive contamination being increasingly problematic)..in the 90's metal particle and BaFe pigments took over tape while disks moved to thin film magnetic layers. As for reel materials and hub evolution..the initial reels were metal and expensive...plastic became normal in the 60's and beyond for the most part..but for master copies or sensitive archival reels..glass or metal were preferred... but changes in the materials were driven by the higher tape speeds,tensions and demands for improved reliability and durability. Hubs in some drives had to be conductive so had carbon black or metals added to them to improve the compressive strength and conductivity. A lot of very innovative but subtle design features moved into tape reels/hubs specifically designed for various transports and industry demands. In addition lubrication and binder changes were common as the needs for the various products in audio, video and digital recording advanced. Hope this helps..but if the interest is primarily in getting a useful detailed knowledge of a particular tape..color is pretty much useless..you need SEM/EDAX and GC/MS and a database of tape analyses to compare to in order to really begin..and then to really know the tape you need DMA/DMTA mechanical analysis, and AFM/MFM surface profiles.....but to my knowledge only IBM had that data and I imagine it ..like so much of that knowledge learned from 1962-2008 is now gone.