finding a min value (real number) & colour cell

You might not need a macro

You can use a formula in conditional formatting

Cell Value is equal to =MIN($D$4:$D$7)

for example

Here's a macro way of doing it

Sub Test()
MinNumber = 1E+18
For Each Cell In Selection
If IsNumeric(Cell) Then
If Cell < MinNumber Then
MinNumber = Cell
MinCellAddress = Cell.Address
End If
End If
Next Cell
If MinCellAddress <> "" Then
Range(MinCellAddress).Interior.ColorIndex = 3
End If
End Sub