Chemistry of transition elements: describe question

A student investigates colour changes of transition metal complexes.

• A pink aqueous solution of $\\ce{[Co(H2O)6]^{2+}}$ turns blue on addition of concentrated hydrochloric acid, forming $\\ce{[CoCl4]^{2-}}$.
• The same pink solution turns yellow on addition of excess aqueous ammonia, forming $\\ce{[Co(NH3)6]^{2+}}$.
• A blue aqueous solution of $\\ce{[Cu(H2O)6]^{2+}}$ forms a blue precipitate of $\\ce{Cu(OH)2}$ on addition of sodium hydroxide.

Describe the origin of colour in transition metal complexes, and explain, with reference to these observations, how the colour of each complex arises from the identity of the ligands and the geometry of the complex.