The compound \([ \text{Ni}(\text{H}_2\text{O})_6 ]^{2+}\) appears green due to d-d transitions in the visible spectrum.
\([ \text{Ni}(\text{CN})_4 ]^{2-}\) is diamagnetic and does not exhibit any d-d transitions, rendering it colourless.
Thus, the correct answer is option 2.
Match List - I with List - II:
List - I:
(A) \([ \text{MnBr}_4]^{2-}\)
(B) \([ \text{FeF}_6]^{3-}\)
(C) \([ \text{Co(C}_2\text{O}_4)_3]^{3-}\)
(D) \([ \text{Ni(CO)}_4]\)
List - II:
(I) d²sp³ diamagnetic
(II) sp²d² paramagnetic
(III) sp³ diamagnetic
(IV) sp³ paramagnetic