Correct me if I am wrong
I think problem is here
Code:

Case $ClientColor="Oranje"

$ClientColor = 0 (as an Integer)
"Oranje" is being converted to an integer, Cint("Oranje"), = 0
0 = 0
return = 1
so what should be done is
Code:

If IsDeclared($client)
	If CheckClient (CStr($Client)) = 1
		? "ClientOK" ?
	Else
		? "ErrorMessage" ?
	EndIf
EndIf