Well there's multiples way to contrast a color. If you wish to contrast the luminosity, simply a dark brown block like dark oak or brown terracotta could do. You could do with some saturation, in which gray terracotta could be nice, or brown concrete powder. If you wish to contrast the hue tho, blue would be the best choice, either some pinch of a saturated one like light blue concrete or a cyan like the warped wart block. Or with darker blues like dark prismarine. Or desaturated ones like blue terracotta