The books didn't have this kind of covers when they were first released! Look:
In both of them the orange cat is Firestar, the famous kittypet that became ThunderClan's best leader and one of my favourite characters.
Vicky (one of the Erins) describes Firestar as her is painted on the second cover:
"Submerged in glorious, vibrant green, Rusty takes his first steps in the forest… Look at that emerald gaze, searing into your soul. Wary, curious, but so full of courage. Oh yes, this cat will go farther than his wildest dreams could take him. Green and orange: the two colours that paint this book most vividly in my imagination, captured here almost to the exclusion of every other colour. And I say, perfect!"