Must Trick To BackColor Textbox

I open form and get parameter in the openArgs

I have a tabular Form
and I want change the color if the ID is aqual.

for example:
I open the form like this:

DoCmd.OpenForm "frmMessageShow", , , "fID=" & Me.fID, , , Me.AssighmentId
[the Me.AssighmentId is the openArgs]

I want in the second form to color the textbox if AssighmentId = me.openargs

I try do it:
If Me.AssighmentId = Me.OpenArgs Then
AssighmentId.BackColor = vbRed
End If

and its not work :mad:

is there is an option to do it?

thanks!