Hot mustard, Brassica nigra, is native to southern Europe, so it seems odd that the ancient Greeks took to this spice fairly late in the game. Apparently, it wasn't until about the second century BC that the Greeks were using the seeds as a flavoring. Before then, they grew mustard only for the greens.
The Romans were more into it. They often threw ground mustard into complicated sweet-sour sauces, above all for birds, fish and wild boar. And when we say complicated, we mean things like this sauce for venison: pepper, lovage, onion, oregano, pounded pine nuts and dates, honey, fish sauce, mustard, vinegar and oil.
They made something like what we'd call prepared mustard too, and served it, as we would, with sausages. In "De Re Rustica," the 1st century writer Columella gave a recipe for table mustard that included vinegar, pine nuts and almonds.
This nutty sort of mustard continued to be made well into the Middle Ages; an almond-mustard sauce recipe was given in a cookbook of 13th century Moorish Spain. Why did people add the ground nuts? Maybe because mustard was too hot for them. Both Roman and Moorish recipes remarked that the addition of nuts tempers the flavor.
And both added that the nuts give the mustard a nice light color. This may sound silly to us, but our own American hot dog mustard is more than a bit similar. It's based on mild B. alba, rather than hot B. nigra, and is brightly colored with turmeric.