Would you mind explaining your question a bit further? Off the bat, I can think that many paintings, both mural and on canvas or wood, have a sort of yellowish colour due to the simple fact that they are several centuries old and have not been restored, cleaned, or taken proper care of.

As a quick example of this, think of the Gioconda, the beautiful painting by Leonardo da Vinci. It has a distintctive yellowish colour that has nothing to do with what the great master painted. This yellowish finish is due to the degradation of the varnishes that finish the painting, that after a few centuries have become degraded or rotten. You also have to consider other elements such as smoke from candles, smoke from tobacco, insects' feces and larvae, etc. All of these will contribute to the degradation of old art. In the Museo del Prado, in Madrid, there is a coetanous copy of the Gioconda that was made in Leonardo's own workshop, as evidenced by the subjacent design and pentimenti, and there you can see what a thorough cleaning and proper restoration from the best specialists can do. If you want to know what Leonardo painted, you would have a more precise idea looking at El Prado's Gioconda.

Here you can see both of the paintings:

Gioconda from the Louvre

Gioconda from the Prado

This is not a unique case, you can check other masterpieces like the frescoes from the Sistine Chapel, and how they looked before and after their restoration. Those frescoes had accumulated several hundred years of dust, smoke from candles, etc. After the restoration, they look much brighter