Still practising cats here. This one was tricky both for lack of chroma and lack of form: a grey fluff-ball basically. I used black only for his eyes and nose, otherwise I mixed chromatic greys using complementary colours.
To achieve fluffiness I used fingers more than brushes to paint his coat. This is worked on rough linen, not the smooth board I usually use. I discovered that rough linen works better if what you want to achieve is fluff.