A blue flame means complete combustion is taking place. It’s not unusual to see a few yellow flickers through a predominantly blue flame, as shown in the photo, this is due to small impurities in the gas and is not something to worry about.
A key warning sign that your gas appliance needs cleaning or servicing are predominantly yellow or red flames or difficultly lighting.
Removing the burners and giving them a good clean can solve most problems but if that doesn’t help call a gasfitter for an appliance service.
Thanks for reading,
Joshua Bell