Soot covering the paintwork from the propane fire?

We have a pyromaster propane fire and we have found that since lighting it for the winter, all of a sudden there is a black layer of soot everywhere, its really bad. The fire was serviced when we moved into the house last year but we have noticed the flames on the fire are quite yellow, we can see blue but it is just as it comes out of the pipe. I have read that a yellow flame is dirty? And can we dismantle it and clean out its jets easily, we know it will cost quite a lot of money to have it cleaned by a company.