Explain how the recessive trait for coat colour is hidden in cats when two parents that are purebred for different fur colour are crossed

When two parents are purebred it means that they are homozygous for the characteristic that's being tested (in this case the fur colour). When they are crossed, the offspring will obligatory be heterozygous, an allele of each type from its parents. So, the trait that comes up will be the dominant trait, while the recessive trait will be hidden.