What you're most likely seeing is the color change of several complexes of cobalt.
First, you have [ Co(H2O)6 ] +2, which is the aqua complex formed when cobalt II is put into aqueous solution. This is a pink color. When you add OH- to the solution, you get Co(OH)2 at first (which is either pink or blue, depending on conditions). This is amphoteric, which means it dissolves in either acid or base, and will dissolve if enough OH- is added, to give the tetrahedral complex [ Co(OH)4 ] -2. This complex is a deep blue.
This information came from Cotton & Wilkinson's Advanced Inorganic Chemistry. You might want to see if you can find a copy of it in your library someplace.