Similarities between Co and Ni
Cobalt and Iron ions are the only ones to produce colored complexes when mixed with thiocyanate ion $\ce{SCN^-}$. If you mix up a dilute acidic solution of Iron(III) Chloride $\ce{FeCl_3}$ and a similar solution of ammonium thiocyanate $\ce{NH_4CNS}$, a extremely dark red solution is obtained, due to a complex containing the ions $\ce{Fe^{3+}}$ and $\ce{SCN^-}$. If you replace $\ce{FeCl_3}$ by a solution of Cobalt Chloride $\ce{CoCl_2}$, you obtain a blue color due to a complex containing $\ce{Co^{2+}}$ and $\ce{SCN^-}$. No other metallic ions are known to produce such colored complexes with thiocyanate ions, at least among the usual ions studied in chemical analysis courses. The blue Cobalt complex has also the property of being soluble in pentanol, where it can be extracted if the initial solution is colored.

Any way it should be mentioned that :

1) The reaction with Iron is extremely sensitive. The red color may still be seen if the concentration of the complex is 1 - 2 ppm. The blue complex made with Cobalt ions is only visible at much higher concentrations. 

2) If this reaction is made with a Cobalt solution containing Iron as an impurity, the solution will turn red, as the blue color of Cobalt will be hidden. In this situation, a couple of milligrams of Potassium fluoride $\ce{KF}$ should be added to the solution. This will produce a colorless complex of $\ce{FeF_6^{3-}}$ and the blue color of the cobalt complex is restored.