Even if there weren't multiple fathers involved, those two absolutely could be brothers. Cat on the left is a red tabby, cat on the right is a brown tabby (genetically, a black tabby) -- the genes behind the two colors are very similar! The only difference in their colors is left-cat has a sex-linked red gene masking the underlying black base color.
Here are some of my cats when they were kittens. They all look very different, their mother is a red/black tortoiseshell but they could absolutely share the same father if he was a gray bicolor tabby.
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Yes on the color. The genetics behind the pattern are complicated and at minimum (there are arguments that there are even more than this) involve a gene for the type of tabby pattern and another gene for agouti (whether or not the pattern is presented). So, say, the mother has no tabby markings and the father is a mackerel tabby -- they could end up with spotted tabby kitten because the mother may have the genes for spotted tabby, they were just hidden.So the pattern came from the father then and the mixed color came from the mother?