You can make armor without vanilla texture, if you extract the armor textures(not as item) from the minecraft jar, then retexture it, then import it to MCreator, you should be able to use it.Quote from ja4nos4ever
Sadly, you can only make armour with the vanilla looks... I really think it's a huge problem but it's not the only one in the program, so it probably won't get fixed anytime soon.
P.S you can also extract as an item then imort that texture for how it looks in your inventory. You could use this to make say a banana helmet in your inventory, but looks like a Batman mask when your wearing it.