[quote user="Barry Lothian"]The problem is I have hard-coded the textbox object name[/quote]
As Jan writes, you don't need to do that. You know which control invoked your subroutine: TB_CustomRed, TB_CustomBlue or TB_CustomGreen. Write your subroutine as PrivateSub ValidateRGB(ByVal value As String) and pass it the value of whichever text box.
VBA Logical Expression Evaluation
[quote user="Barry Lothian"]IfNotIsNumeric(.Value) And.Value <> vbNullString Then[/quote]
Unlike most other computer languages, VBA evaluates all expressions in a logical statement. In your case, it's possible to have an empty string that may cause IsNumeric(.Value) to throw an error. The following avoids that error...
If .Value <> vbNullString Then
If IsNumeric(.Value) Then
Dim number As Double
number = Dbl(.Value)
Else
' Not numeric
End If
Else
' Empty string
End If