The Heliopan 40.5mm KB-18 Color Conversion Glass Filter is blue. It is used to balance daylight film under artificial, tungsten or standard (household) incandescent lighting.
We have all taken indoor photos (with traditional film) that turn out yellowish. A portrait under this kind of scenario usually leads to less than desirable results. In order to bring back a reasonable color balance to the image, a KB-18 is required.
In some instances, that "yellowish" tint may be desired to provide a somewhat "old style" look to the photo, but in general, it tends to be distracting and unflattering to most subjects.
Heliopan Lichtfilter-Technik of Munich, Germany, has changed their black anodized brass filter rings on most Heliopan non-rotating filters to their "slim" mounts. These slim mounts are made of black anodized brass stock and have so little rim in front of the lens (5.4mm) that they will not vignette with lenses as short as 21mm.
Heliopan offers a wide selection of sizes, screw-in, series, bayonet, square and rectangular filters including digital, video, center weighted, graduated, wide angle, Kaesmann polarizers, drop-in polarizers, etc.