How can I make a cell's font transparent?

I've found a work-around that doesn't change the font color, but effectively removes the text from the cells. Adjust the cell number format to Custom, with a value of ;;;.

While I'm not aware of any worksheet functionality to match a cell's font color to its fill color (other than manually), it's very easy to do with a macro. The first macro below changes the font color(s) in the cells of a selected range to match their fill colors. The second returns the font colors back to the default black.

Sub HideFont()
    Dim cell As Variant
    For Each cell In Selection
        cell.Font.Color = cell.Interior.Color
    Next cell
End Sub

Sub UnhideFont()
    Dim cell As Variant
    For Each cell In Selection
        cell.Font.Color = 0
    Next cell
End Sub

To install the macros, select Developer / Visual Basic from the main ribbon and then choose Insert / Module from the menu. Paste the code into the edit pane that opens up. The macros will appear in the macro list accessible by choosing Developer / Macros from the main ribbon. Just select with the mouse the range you want to modify and choose the macro you want to run.