Using a marble stone as a palette

Sounds to me like that will serve nicely as a palette for oil painting, particularly for making paint (if a little short on the narrow dimension). ​If the slab is Composite Marble, an engineered stone material, it will be less absorbent than natural stone. Natural marble will take up some paint and medium, and become less “thirsty” through use. If it’s too absorbent, rub in a small amount of linseed oil. You will find that the stone will take up a stain over time, which should serve nicely as a warm neutral background for color mixtures. 

Marble slabs were universally used in print shops for mixing inks. Years ago, when I was in college, a professor found one abandoned in the alley behind his studio after a nearby basement had been cleared out. We students were all very jealous, and telling the tale now makes me feel a slight pang of envy.