These four metals are treated to give more color, or more resistance. Typical treatment include anodization - an electrolytic process - or coating - physically adding a layer of another material.
Aluminium can be anodized to bold bright colors. It is one of my favorite chainmail jewelry material. Anodized aluminium is not produced like other anodized metals. While metals like niobium and titanium are anodized by applying a strong voltage to the metal (the thickness of the oxide layer giving a wide range of colors), anodized aluminium must first be subjected to a similar process (which produce a colorless oxide layer) and then dyed to color the oxide layer.
Niobium can be anodized to a wide selection of color and is hypoallergenic. It is what I would call a ''semi-precious'' metal. Along with titanium, it is much more expensive than brass, bronze or steel, but not as costly as silver. It is a very fun metal to work it, as it normal temper is very close to dead soft sterling silver. Being used to working with aluminium, which is very springy and hard, it makes for a nice change of pace.
Titanium, like niobium, can be anodized to a wide selection of color and is hypoallergenic. It is a very hard metal to work it, because of its hard temper which makes it hard to bend and subject to easy breakage when working it. In the case of rings, this metal requires special pliers in order to be able to close a fairly standard 18 gauge ring.
Copper wire, covered with a layer of colored plastic is available in tons of different colors, and is very useful to create wire components. I use a lot of silvered copper wire, which consist of a copper wire plated in silver, then covered in a plastic layer, mostly for earring components (never the hook). This gives very bright colors.