1) You did not include the last important bit of code where you select
the range  to be evaluated by Color_Cell.

So I am going to assume that your defined Range consists of more than
one cell, and it is defined as follows:
Dim Cell
Cell = Selection.Address

Since you have multiple cell  selected in your range, when you try and
evaluate Color_Cell, you can only evaluate the first cell in the range.
It would be better to loop through all cells  in the range Cell, and
evaluate each one on its own.

2) error  13 is a Type Mismatch error. For more information, search for
"trappable errors" in the VBE. That will explain it very well, and will
provide some insight on how to avoid it.

If you are only wanting to apply this to one cell  at a time, then in your
first macro  put, immediately following the "Private Sub... " line

If Target.Count >1 then exit sub

That was the simple logic  that I wasn't
able to come up with. I love it when the simplest answer is the most
elusive!

My 1st observation would be to exchange your if/then logic  to case/select.

vGetCellValue = Cell.Value
Select Case vGetCellValue
Case "g"
Cell.Font.ColorInde x = 4
Cell.Interior.ColorIndex = 4
Case "b"
Cell.Font.ColorInde x = 1
Cell.Interior.ColorIndex = 1
Case "r"
Cell.Font.ColorInde x = 3
Cell.Interior.ColorIndex = 3
Case Else
'default option
End Select