I accidentally applied a colour search filter on the Wardrobe, by clicking on a coloured square under the item name, and there's no indication of how to remove it; clicking it again doesn't seem to work. How to I get rid of it to see my full collection, without saving my outfit and reloading the Wardobe entirely?

can't. You can workaround it because very few items have just one color, just lock yourself into another color that it available. Usually easier to save and quit though.