In 1994, the company purchased all rights to the Garfield comic strips from 1978 to 1993 from United Feature Syndicate, although United still holds the original black-and-white daily strips and original color Sunday strips. The full-color daily strips and recolored Sunday strips are copyrighted to Paws as they are considered a different product.
So maybe it's down to copyright law that the strips have differing colors (and that the coloring is chosen to be completely over the top to ensure it can be claimed to be different enough from the other versions owned by other companies?)