Say if I wanted to add an item that supports both stony and metallic categories. I wanted my item in stone to use not only a different color, but a different image than if it was created with steel. My main issue is that I want this to be selected under only one icon in the menu. How can I go about doing this?
I'm assuming you mean something along the lines of a box with a smooth texture to it if made of steel and a cobblestone-esque texture if made of stone. In which case if you are I don't think you can do two textures with one thingdef. Even if you can you might be better off with just having separate thingdefs.
That's unfortunate if I can't. I feel like it kind of needs to be done in this situation if I'm going to implement two types of textures.
If you don't mind my asking what is it your texting?
Because if it's a weapon, or something that'll just sit in a warehouse that extra level of detail will just translate into extra RAM usage in a 32 bit game. While it's nice, you are ultimately talking about something that won't be easily seen (in that case)
Walls. Walls made in metallic category uses different textures than stone. I wanted to make textures that blend with both types as well. I didn't want to clutter the menu and make it confusing to players.