Was browsing through Horenstein's Colour Photography book and found this interesting passage about color-polarizing filters that produce varying intensities of a colour (or sometimes multiple colours) as you turn the filter.
Yet I haven't been able to find any information about these online much less any available to buy.
If you have a birefringent material between polarizers you can get colors depending on the angle. Never heard of this being used in photo filters before.
I've seen these many decades ago, and wasn't impressed. At that time, anything that involved two polarizers and a birefringent sheet before multi-coating presented difficulties that may have precluded image quality adequate for advanced photographers. The sample I once owned appeared to be cheaply made. I never tried to photograph with it.
I have one, it's a yellow-blue one. I haven't used it much but it is a pretty cool item, some unique results. they are still out there and available last time I checked, not super common but not extinct.