You can use the characters(start, length) property to do this. You can also store the text in a string and loop on that, which will be faster when you work with many cells. Here is an example:
Sub RedText()
Dim i As Long
Dim text As String
text = Cells(1, 1).Value
For i = 1 To Len(text)
If IsNumeric(Mid$(text, i, 1)) = True Then
Cells(1, 1).Characters(i, 1).Font.Color = vbRed
End If
Next
End Sub