Tom Kitten, brother kitten (brown shade brown mackerel tabby)
Miss Moppet, sister kitten (warm-grey-brown shade brown makerel tabby with white)
Mittens, sister kitten (warm-orange-brown shade brown mackerel tabby with white)
Tabitha Twitchit, Mittens, Moppet, and Tom's mom (van patterned black and white)
Cousin Ribby, Tabitha's cousin (warm-orange-brown shade brown mackerel tabby with white)

Mittens and Moppet have low-grade white spotting, Tom doesn't appear to have any white spotting, and all three are brown mackerel tabbies. Considering that Mittens, Moppet, and Tom's mother is a black cat with high-grade white spotting, their unknown father would most likely be a brown mackerel tabby without white spotting. Tom would likely carry a gene for white spotting, but not express it.

Note: Brown/black tabby is the agouti or tabby pattern version of black and blue/grey tabby is the agouti version of grey/blue. Mackerel tabby is a tabby with stripes. Broken mackerel tabby is a variant of the mackerel tabby that has broken up stripes. The warm brown tabby appears brown, orange brown, red brown, dark brown, sandy brown, or yellow brown while the cool brown tabby appears gray, dark grey, or cool grey. Between cool brown tabby and warm brown tabby would be grey-brown, brown-grey, dark grey-brown, and dark brown-grey.

Tom Kitten, Moppet, Mittens, Tabitha Twitchit, Cousin Ribby, The Tale of Tom Kitten, The Story of Miss Moppet, and The Tale of Samuel Whiskers: Owned and written by Beatrix Potter