Yeah, the milky white carts were Japanese rental carts. Other than that, I don't know if there's any specific meaning to a cart color. Many originals are known to come in odd colors, like blue, clear, bright teal (Mark of the Wolves), or even yellow (Matrimelee).
Some carts, like your Metal Slug and Super Sidekicks 3, will have their serial numbers scratched off or blacked out. That means the cart was originally purchased by an arcade operator. From what I understand, the arcade op's could get in trouble back then for reselling MVS carts, so they often got rid of the serial numbers so no one could trace the cart back to them.