FormatPercent
returns string in format "20%"
and you can't comare this string with numbers.
Try this one instead:
Dim res As Double
If Me.netto.Value <> "" And Me.vat.Value <> "" And Me.netto.Value <> 0 _
And IsNumeric(Me.netto.Value) And IsNumeric(Me.vat.Value) Then
res = CDbl(Me.vat) / CDbl(Me.netto)
Procent_check.Caption = FormatPercent(res, 2)
If res >= 0.27 Or res <= 0.15 Then
Procent_check.ForeColor = vbBlack
Else
Procent_check.ForeColor = vbRed
End If
Else
Procent_check.Caption = ""
End If