There are two causes of yellow tips and soot on cookers and other gas appliances. The appliances haven't been converted to LPG from natural gas (methane) using the appropriate LPG jets, either supplied with the appliance or purchased separately from the manufacturer, and have just been connected straight to butane / propane cylinders or a propane tank supply, natural gas (methane) and LPG have different calorific values and heat outputs and are incompatible i.e. LPG appliances are not designed to run on natural gas (methane) and appliances set for natural gas cannot be run on LPG either. The other cause is inadequate air set for the burners, as LPG butane gas and propane gas need significantly more air to burn cleanly, without yellow tipping and / or sooting, this can be done usually by aeration screws, an adjustable air shutter, or an adjustable air collar in the appliance which can be set for the gas you are using.