go to technical inspection fill the change/secondry color form or get it typed.take ur car to the officers wearing brown uniform.show them ur car and the form explain them u will only be painting ur hood.once they sign and stamp the form pay the necessary fees.take ur car have it painted,bring it back to the technical inspection for the officers to inspect.if satisfied (which i think they will be coz u only painting the hood)they will stamp, approve.go inside give them ur old istimara along with the color change form pay 100 riyals and the will mention ur secondary color on ur istimara.thats it.
i painted my hood and the back for my car as black and the remaning was grey.
hope it helps
goodluck