Why does the colour of cobalt hydroxide depend on the base?

You get different colours because different substances precipitate. If you add NaOH to a solution of a Co(II) salt,[math] \text{Co(OH)}_2 [/math]is formed. Ammoniumhydroxide does not exist (you can not isolate this compound). read more

As ammonia is a base ammonium cations and hydroxide anions are formed: [math]\text{NH}_3 + \text{H}_2\text{O} \rightleftarrows \text{NH}_4^+ + \text{OH}^-[/math] Now, in principle the hydroxide anions could directly form [math]\text{Co(OH)}_2[/math], so that the same precipitate would be formed. read more

So maybe the species after blue cobalt II hydroxide has been left standing in air is more and more of the hexaaqua species, which is pink. So after standing the species is in fact a mixture of the blue and the pink species. read more

Cobalt(II) hydroxide decomposes to cobalt(II) oxide at 168 °C under vacuum and is oxidized by air. The thermal decomposition product in air above 300 °C is Co 3 O 4. Like iron(II) hydroxide, cobalt(II) hydroxide is a basic hydroxide. It forms [Co(H 2 O) 6] 2+ in acidic aqueous solutions. read more