Conditional Formatting on the Basis of Cell Shading

Hi All,
I am a newbie to this forum...But I found that it is an ocean of information related to ms-excel...
Could anybody help me to sort out: how we use the in-built tool of excel to make conditional formatting. What I want is "if(the color of a cell/range of cells is red(for example), make it 'green'). It means not on the basis of the cell content, only on cell color...What my aim is, I want the red shaded cells to be green..that's all.. not using vba. It seems to be simple..but, for me, difficult. I'm using excel 2007.

Please help.