Tiron (And then, the mighty Tiron bound the iron tightly.)

Tiron is a metal ligand and can be used in colorimetric metal assays.

Transition metal complexes tend to have the happy property of having their frontier orbitals separated by energy characteristic of photons of visible light - that is, they end up having pretty colors. I've never actually used it, but Merck claims complexes with different metals that span essentially the whole visible spectrum - iron gives blue, copper greenish-yellow, titanium orange, and molybdenum yellow.

For another example of a colorimetrically useful metal ligand, see my entry on that wonderfully named staple of undergrad analytical chemistry labs, eriochrome black T.