Glazes: There are literally hundreds of glazes they could and probably would have been using at this time. I have the same reaction as to the metals: redundancy without gameplay is a little frustrating at best. However, a glaze for each type of color option you might want for decoration might be reasonable and meaningful. A wide variety of glazes is not high tech either. As long as you have a hot enough kiln, it's basically the exact same process for everything - grind stuff up (various minerals), mix with some clay and perhaps whiting, and dip or brush then fire. So virtually every glaze we know about today would be reasonable, except for lab synthetic substances.
At the cost of a little realism, I'd suggest bringing the dye mechanism to glaze. Since the Dyer's Workshop is not in the raws, it isn't constrained by what we can mod. Allow your dwarves to dye the glaze at the Dyer's Workshop before applying it to the earthenware, and viola you have colorful pottery. Of course, this completely ignores how the dye might break down at high temperatures, but setting dyed clothing on fire doesn't seem to change the color, so at least it's consistent.
Why take such a weird, clearly-not-realistic-at-all approach, when you could simply add actualy glaze materials from the many abundantly available actual minerals, etc. in the game? In fact, I've already written up a list myself for a mod I was considering awhile back of mostly-realistic glaze recipes:
Glaze Colors:
Black - (cobaltite and iron), (iron and fuel)
Navy - cobaltite
Blue - rutile, tin, (cobaltite and feldspar)
Cyan D - (copper + ash), iron, chromite, tin
Cyan L - ash, tin, chromite, (iron + feldspar)
Green D - iron, garnierite, chromite
Green L - copper, chromite, ash, (iron + flux)
Ruby - (chromite and lead), (feldspar and copper)
Red - copper, iron, pitchblende
Purple - (pyrolusite and ash) -- purples and magentas MIGHT also be possible with generic ultramafic rocks (dunite, etc. There actually aren't many in-game. Hornblende is one. Presumably "semi-molten rock" stands in for ultramafics, and below that, there is HFS instead of a normal mantle) + cobalt glazes. NOT celadon-type blues, though.
Magenta - pyrolusite
Brown - (chromite + zinc), iron, rutile, lead
Yellow - (chromite), (iron + talc), iron + flux
Gray - garnierite, (iron + fuel)
L. Gray - iron or rutile
White - feldspar
"feldspar" refers to any felsic rock, pretty much. Such as quartzite, granite, rhyolite, porphyry, obsidian, etc. Although generally you'd want to restrict yourself to only the MOST felsic rocks, like quartzes and orthoclases.
You wouldn't probably want ALL of those. It's just a list of many options from which a balancing game designer might choose. There would be a few more too.
(This list is abstracting away oxidizing versus reduction firing environments, with different recipes above being in either category. Black with iron and carbon would be highly reducing only, for instance, while iron-based reds are oxidizing kilns)