Edit: 12 comments told me that cats can have purple eyes,

Cat appearances are difficult to come up with if you aren't familiar with the Allegiances. But, if you weren't familiar with the Allegiances, why would you be making a Warriors fanfiction anyway?

All sarcasm aside, the pattern of a cat's fur should be similar to the pattern of furs of cats in real life: tabby, tortoiseshell, spotted, etc.

The colors and patterns, on the other hand, should also be realistic: blue (as in bluish gray), dark or light gray, black, white, tawny (sandy colored), ginger/orange, tortoiseshell, tabby, pale, reddish-brown, speckled, with colored flecks, etc.

Eye colors also follow in suit with the fur colors and fur patterns. They include: blue, amber/yellow orange, clouded if blind, golden, red if you really think that albino cats should be in the series (it's original, but I dunno), and green. They should not be black or purple, nor rainbow. (UPDATE JULY 3, 2018, TUESDAY: I am aware that some cats may have purple eyes in real life as several of the comments point out. Please keep in mind that I am not a complete expert on all things cat-related, so I did not know that when I originally wrote this chapter.)

Cat's tails can be fluffy, short, long, or not there at all. Manx cats are the only cat breed without tails unless the cat lost it from an accident (although cat breeds aren't mentioned in the series).

(You guys have also said that Manx cats do in fact have tails, but they are very short.)

Tails should not be unrealistically long (say, more than a tail-length in the books; look at the chapter that informs about the cat language in the Warrior books), or have unusual colors. (No, you cannot make your cat have rainbow tails) The tails cannot be as long as a house or dragon-like per se.

The nose color can be pink, whether it be light or dark pink, or other cat nose colors such as velvet. This isn't usually an important detail when writing the Allegiances, but if you wanted to write a cat's nose color into the story, then make the nose pink or another realistic color. Just don't make it orange, red, blue, purple, green, yellow...you get the idea (Another thing the comments have told me is that cats have nose colors specific to other things related to their appearances.)

And finally, extra additions to the cat's appearance, such as scratches, scars, broken limbs, or broken jaws. Cats who have injuries should have the injuries be realistic. You can't just have a cat that was hit by a monster, completely flattened, and is just fine.

A cat's injury shouldn't be massive. For example, don't make your cat have a body-long scar that goes from his/her right eye to his/her tail. That is not realistic. And do not have your cat be burnt to a crisp. That is also not realistic and the cat would have died. I'm sorry, but it's true. Cats cannot be completely burnt of every piece of fur on their body and survive. That is just unrealistic and completely out of place for cats who live in the wild.

All in all, just make sure your cat's appearance is realistic, MAKES SENSE, is RELEVANT to the PLOT, and doesn't include the following: (rewritten)

-rainbow colored fur (pink, neon or really REALLY bright yellow, green, unusual shade of blue, purple, or rainbow)

-rainbow colored eyes (pink, magenta, neon colors, black, rainbow)

-extremely long claws (reaches more than the top of the ground while not unsheathed) (unless it is Tigerclaw-like or something)

-extremely long tails (as long as a house or something like that is just crazy)

-extreme injuries that cats survive (cat burnt to a crisp and surviving)

-extremely long teeth (touching the ground; sabertooth-like) (unless it's Stonetooth or something like that)

-rainbow nose color (red, orange, yellow, green, blue, purple)