I have been sent a number of hair samples from Norwegian Forest cats who have avery unusual color. I have been told that they are born looking like light/warm back tabbies (or brown tabbies- depending on registry policies, these are tabbies with black markings on a brown background), but as the get older the color gets lighter and hotter, until they resemble cinnamon tabbies.

So far, my analysis indicates that these ARE black tabbies. The hairs are black at their darkest areas, near the tip, but they do an unusual slow fade through brown and reddish colors ending in a long pale root. I wonder of this results from a mutation in the agouti gene.

Two other observations, two of the cats have hair coloration that almost looks like dilute (blue), with irregular distribution of pigment, but nowhere near as irregular or light as a typical dilute. Also many of the cats had strikingly transparent hair tips, not at all characteristic of your typical tabby, though typical of Russian Blues.

Normal black tabby hairs:

 

  Normal blue tabby hair:

 

Serial Pictures down the length of an "X" color NFC from transparent tip at top right to a light area about 1/3 down the length of the hair shaft,  This is one of the two cats whose samples looked somewhat similar to a dilute (blue) tabby.  Note that the hair goes from a very transparent tip, through a region of black pigment, then the pigment becomes brown.  Farther down the hair shaft the color becomes almost red.  I have not seen hairs like this before.  These are 100X magnification through an Olympus AX-70 microscope, photographed on a white background with external overhead light.

Here are later pictures of a spine-line hair under better background/lighting (on graycard) showing better color.  At the top is the transparent tip shading into black.  Below is black shading into brown, then below that the next color, coppery gold, and finally the long creamy colored hair base.  This hair does not exhibit the dilute-like hatching of the hair above.

Another "X" color cat hair.  This cat, like most of the X color cat's whose hairs I looked at, had the transparent tip seen above, but not the pronounced irregular pigment desposition that looks almost dilute.  Notice that this hair is much more evenly colored.  The top shows the transparent tip, and goes into the black region.  The bottom shows the black region heading into brown.  It then passed through brown into a reddish color almost like cinamon, and finally to a creamy color near the root (not shown).  These photos are 40X but otherwise the same as above.