If you have none how can you say they are specifically brown? You could say they are any color then which makes them being just 1 specific color not true. Your non-existent cats aren't brown, they are every color or even no color.
Maybe even more accurately they aren't brown, they are an undefined color.
I'm not really satisfied saying that the characteristics of something that doesn't exist can be anything. I am satisfied saying the characteristics are undefined though.
The article states that "this question was originally set in a maths exam, so the answer assumes some basic assumptions about formal logic." This is a funny way of phrasing it, but should make it sufficiently clear to any perceptive reader of The Guardian that they are supposed to ignore the colloquial interpretation of the phrase.
At any rate, it would hardly qualify as a puzzle if the answer was so obvious
Maybe even more accurately they aren't brown, they are an undefined color.
I'm not really satisfied saying that the characteristics of something that doesn't exist can be anything. I am satisfied saying the characteristics are undefined though.