Or another solution is switching the arguments in each case.
In that way it is $ClientColor that wil be converted implicit.
Code:

If NOT @LOGONMODE
	Break ON
EndIf
Dim $SO
$SO = SetOption("Explicit","On")
$SO = SetOption("NoMacrosInStrings","On")
$SO = SetOption("NoVarsInStrings","On")
$SO = SetOption("WrapAtEOL","On")

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

Function CheckClient ($ClientColor)
	Select
		Case "Oranje" = $ClientColor
			$CheckClient=1
		Case "Groen" = $ClientColor
			$CheckClient=2;ABS(1-(InGroup("\\" + @WKSTA + "\Administrators")))
		Case 1
			$CheckClient=0
	EndSelect
EndFunction