The presence of a partially filled d subshell on the metal is usually necessary for a complex to exhibit colour. Most transition-metal ions have a partially filled d subshell. Ions that have completely empty d subshells (such as Al³⁺ and Ti⁴⁺) or completely filled d subshells (such as Zn²⁺, 3d¹⁰) are usually colourless.
Your choices were Cu, Cd, Ni, Cr, Co.