The clay is the color. Clay is the warmer of the two, a little pink at the rim when the light is low. Slate is the same body with a darker grog, fired in the same cycle. Neither is painted. If you chip a foot, you see the same tone all the way through.
The interior is the only glazed surface, and it is there so fruit and wet stems do not stain the wall. The outside stays dry to the hand. We like that a bowl can sit on a table for a year and still look like a thing that was made, not finished.
If you are choosing between the two, pick the one you will leave out. A basin in a cupboard is just a bowl. A basin on the table becomes the place keys and lemons agree to land.