Apart from certain decorative gas fires, appliances should burn gas with a blue flame. A sluggish yellow flame is a sign of incomplete combustion and the formation of toxic carbon monoxide (complete combustion produces pretty much only carbon dioxide and water vapour).
If your boiler has a sight glass that allows you to look at the flame, take an occasional look to check that the flame is blue, sits on the burner and is not lifted off.
If you see a yellow flame, get the boiler serviced promptly. If the flame picture is good, have a routine annual service done as will almost certainly be recommended by the manufacturer. Note that many modern high efficiency boilers have totally enclosed burner and there is no visible flame. Only a flue gas analyser can reveal how well the boiler is burning gas.