On my vet's advice, I've changed my cat's food from Iams dry to Royal Canin Renal LP (wet food). Since doing so, my cat's poo's have changed from a light brown to a very dark brown. Is this normal?

He's also on prednisone at this time to help with a slight "thickening" of one of his intestines, so perhaps its the medication causing it?

I just want to make sure this is normal...