My gas fired steam boiler is showing a yellow flame, is this safe or is it a sign of a problem

I have a gas fired steam boiler and I am concerned about seeing yellow flames is it a sign of a blocked chimminey

When a flame changes colors, it is caused by some form of contamination. That's not nearly as bad as it sounds. For example, if you hold a piece of copper (e.g. a penny) in a flame, it will turn green. It's completely natural. Different things burn at different colors. If you burn platinum, it burns white (be careful, this is more damaging to your eyes than staring at the sun is). When jewelers are working with platinum, they wear special goggles that have lenses that are literally made of gold, pressed so thin you can see through it like glass (I KID YOU NOT!). Anyways, when it's not lit, just scrub it down and rinse it thoroughly. Do not clean it with anything flammable. Just use dish soap and a paper towel, and rinse well. That should restore the flame to its normal color. If that doesn't work, then it's probably burning the metal that it's made of, which is of no danger, just remember not to touch it for a while after it's off as it will be very hot.

Your not looking at your pilot flame are you? This is supposed to burn to light the burner when it kicks on.