You are a pixel-magician. They look fantastic. They do have a lot of color though, did you test them ingame? If the game applies "dark grey" for steel over your already dark grey armor, it might be hard to see.
Thanks. 
The colors are the "ideal" target look, and they will certainly still need to be adjusted before getting them ingame (along with transparencies and backgrounds). And nope, haven't tested anything yet.
I still don't understand how the overrides actually will work, but I'll come to that when I have all the graphics done. I think mifki implemented color overrides as well.
Speaking of TWBT, have you had much luck with your experiments? I'm curious about the building sets in particular. Are fully unique tiles for each building possible or do they still share tiles? I'm wondering how I should tackle things like siege engines or workshops. Not to mention tree leaves and how they differ for different species (pine vs. palm vs. mahogany, for example).
But yeah, one thing at a time. As I've said before, I really am not very good at the coding part.
Since you asking for TWBT info, here is what i know so far:
1. Current version of plugin allows you to have unique tiles for every building, but same tiles in same building will share same tile number if overridden
fricy already described it well
2. Tile numbers that become free to use after addition of TWBT support (for vanilla raws):
0, 18, 23, (24, 25, 26, 27 - good to use as ballista arrowhead, since other DFHack plugin actually uses them on map), 28, 29, 41, 56, 57, 81, 86, 113, 117, 121, 122, 129, 130, 131, 132, 133, 134, 136, 138, 139, 140, 141, 144, 151, 160, 161, 163, 166, 171, 226, 244, 245, 252, 255
3. Speaking about trees: If you want completely unique branches/twigs/etc. – assign them to default wall tiles and use overrides (considering, you are out of free tiles). Behaviour of leaves is quite strange. By default, leaves on tree use tile taken from d_init.txt (TREE_BRANCHES), but if you go to tree raws, you'll find [GROWTH:LEAVES] sections, containing line like [GROWTH_PRINT:
0:
6:
2:0:0:ALL:1] (this taken from plant_new_trees/ABACA), where
green numbers control colour (as usual),
orange number controls tile of this particular growth taken as item (and i don't know how "take leaves as item" works), and
red one controls it's tile while on a tree. Default value is 0, so game itself overrides it to TREE_BRANCHES tile from d_init (and that's quite unusual behaviour), but you are free to set any number you like.
Added: same works with flowers/fruits, with only difference is that usually they have tile different from 0
