Yeah, I imagined that. That way you can only play with contrast and I imagine you must exaggerate the lights and darks in order to have some of that translated to the game color overlays. With different colors you can have "subtle" variation in brightness that gets helped by the difference in hue. If the object is monochrome though... I bet things like the subtle brightness in things like the plain octogonal shields would get lost.
Is there any way I can test the color effect outside of the game?. I mean , something like aplying a colored overlay layer in photoshop over the tiles or something like that. I've tried with an overlay layer but that doesn't seem right. I guess the game works in a more complicated way.
Different colors for different parts... would be awesome but... is it even feasible?. I mean, the object is made with one command (i.e. make iron sword) so all the sword is made from iron. Thinking about it, I suppose you're referring to - through twbt - tell the game to color the grey parts but left the colored ones intact, or something like that right?
In Gnomoria for example, you make weapons and armor from different parts that can get different materials (this wood for the handle, that metal for the blade...) but that would be a nightmare in DF...