[AccessD] Instant If Then Else

Can I make these changes the instant the User enters a text box, please?

If Not IsNull([txtFun]) Then

        Me.txtFun.BackColor = vbRed
            Me.txtFun.ForeColor = vbYellow

                Else

            Me.txtFun.BackColor = vbWhite
        Me.txtFun.ForeColor = vbBlack

    End If

I mean as he types "y" for "yes", or anything at all? The above doesn't work
till the record changes. 

Cheers paul